The Quest for Low Work Function Materials: Advances, Challenges, and Opportunities

Abstract

Low work function (LWF) materials are essential for enabling efficient systems’ behavior in applications ranging from vacuum electronics to energy conversion devices and next-generation opto-electronic interfaces. Recent advances in theory, characterization, and materials engineering have dramatically expanded the candidates for LWF systems, including alkali-based compounds, perovskites, borides, nitrides, barium and scandium oxides, 2D materials, MXenes, functional polymers, carbon materials, and hybrid architectures. This review provides a comprehensive overview of the fundamental mechanisms governing the work function (WF) and discusses the state-of-the-art measurement techniques, as well as the most used computational approaches for predicting and validating WF values. The recent breakthroughs in engineering LWF surfaces through different methods are discussed. Special emphasis is placed on the relationship between predicted and experimentally measured WF values, highlighting the role of surface contamination, reconstruction, and environmental stability. Performance, advantages, and limitations of major LWF material families are fully analyzed, identifying emerging opportunities for next applications. Finally, current and fundamental challenges in achieving scalable, stable, and reproducible LWF surfaces are considered, presenting promising research directions such as high-throughput computational discovery and in situ surface engineering with protective coatings. This review aims to provide a unified framework for understanding, achieving, and advancing LWF materials toward practical and industrially relevant technologies.

1. Introduction

The work function (WF) of a solid defines the minimum energy required to remove an electron from its Fermi level inside the material to the vacuum just outside its surface [1]. As such, it is a key parameter governing surface electron emission [2], charge injection/extraction at interfaces [3], and interfacial alignment in devices formed by multiple heterostructures and/or junctions [4].

Low work function (LWF) materials are essential for technologies where efficient electron emission or collection is needed: vacuum electronics (e.g., field emission transistors) [5,6], photocathodes [7], catalysis [8], perovskite and organic opto-electronic devices [9], thermionic energy converters (TECs) [10], and more.

From a fundamental perspective, the requirement for LWF is not merely incremental but reflects intrinsic physical constraints imposed by electron emission laws. In thermionic-based devices, the Richardson–Dushman relation introduces an exponential dependence of emission current density on the WF, implying that even a reduction of 0.3–0.5 eV can translate into some orders of magnitude enhancement in emission or, equivalently, a substantial reduction in operating temperature [11]. For example, reducing the work function to values below ~2 eV enables efficient emission at temperatures compatible with reduced thermal stress, longer device lifetime, and broader material compatibility in TECs and other vacuum microelectronic devices [12]. Beyond standalone electron emitters, LWF materials play a central role in heterointerfaces, where charge injection, extraction, and band alignment critically determine device performance. At metal–semiconductor, metal–oxide, and electrode–organic interfaces, the effective injection barrier is governed by the relative alignment between the electrode WF and the adjacent material’s band edges. High WF can lead to Schottky barriers or injection-limited transport, whereas LWF electrodes enable quasi-ohmic contacts, reduced contact resistance, and enhanced carrier transmission. This requirement is particularly stringent in emerging heterostructured devices (e.g., photocathodes, catalytic, and opto-electronic systems), where interfacial barriers can dominate over bulk transport. In these systems, reducing the electrode work function below ~2 eV can qualitatively alter interfacial band bending, suppress barrier formation, and stabilize efficient charge transfer without relying on heavy doping or aggressive chemical activation. Consequently, WF engineering at heterointerfaces is not merely an optimization parameter, but a fundamental enabler of scalable and low-loss device architectures.

Historically, materials, such as tungsten coated with alkali metals [13], barium-oxide impregnated cathodes [14], and lanthanum hexaboride (LaB₆) [15,16], have served as standard emitters thanks to WFs in the ~2–3 eV range and acceptable stability under high temperature and vacuum conditions. However, modern device demands—including flexible substrates, ambient-air operation, low-temperature/-field emission, solution-processable fabrication, and integration on semiconductors (e.g., silicon or III-V alloys) or large-area substrates—push the requirement toward even lower WFs (2 eV, ideally approaching ~1 eV) together with environmental robustness, process compatibility, and scalability.

In response, two major research directions have emerged: (i) high-throughput computational screening [17] and theoretical design of new LWF candidate materials (e.g., perovskite oxides predicted to have a WF of ~1 eV [18]) and (ii) experimental engineering of interfaces/coatings [19,20], doping [21], nanostructring [22], and protective overlayers to achieve low WF and enhanced stability [23]. Nonetheless, a persistent trade-off remains: achieving ultra-low WF often comes at the cost of chemical/ambient stability, and/or processability may limit ultimate WF performance. Additionally, measurement/benchmarking of WF across studies lacks standardization.

Several comprehensive reviews have addressed the fundamental physics of the WF, its measurement techniques, and general engineering strategies. Rather than duplicating this coverage, the present work adopts a complementary perspective that is explicitly centered on strategies for reducing WF, classifying materials, and considering device-relevant constraints. This review, as summarized in Figure 1, aims to provide a critical overview of the main results achieved in the WF reduction, covering fundamental mechanisms of WF, material families, measurement and computational approaches, stability and scalability issues, emerging applications, and open research directions toward deployable and application-ready LWF materials for next-generation technologies.

2. Theoretical Foundations and Mechanisms

First, for clarity, it is important to distinguish between several related but conceptually distinct quantities often discussed under the umbrella of “work function”. The WF is defined as the energy difference between the vacuum level (the potential energy of an electron just outside the surface) and the Fermi energy inside a solid under equilibrium conditions. For semiconductors, the electron affinity (EA) is the difference between the vacuum level and the conduction band minimum (CBM). The vacuum level lies below the CBM in materials exhibiting negative electron affinity (NEA), enabling electron emission without an intrinsic surface barrier. In practical devices, however, flat band conditions are often unreal. Therefore, the relevant quantity is often an effective emission barrier, which may differ from the true WF due to band bending (BB), surface dipoles, electric fields, or illumination-induced effects. For instance, under optical excitation, surface photovoltage (SPV) can transiently shift band edges and vacuum levels, leading to time-dependent reductions in the effective barrier that do not correspond to WF values measured at equilibrium. Figure 2 schematically summarizes the key energetic quantities relevant to WF engineering and electron emission discussed in this paper. In particular, it illustrates equilibrium WF concepts and non-equilibrium for effective WF arising from BB and illumination-induced SPV. Distinguishing among these quantities is essential for semiconductors when comparing reported ultra-low WF values across different measurement techniques and operating conditions.

The WF depends both on bulk electronic structure (i.e., Fermi level position, band structure) and surface phenomena (i.e., surface dipoles, adsorbates, reconstructions, termination, morphological features). The key mechanisms influencing WF include the following:

• Surface dipoles and adsorbates: At the material surface or interface, a dipole layer (for example, due to adsorbed alkali metals, molecules, or an atomically thin coating) shifts the vacuum surface potential, thereby lowering (or raising) WF [24,25]. Carefully engineered surface dipoles can reduce WF down to 2–3 eV.
• Band structure and Fermi level tuning [26]: Materials with high carrier concentration (metallic conduction) and Fermi level close to the conduction band minimum (CBM) naturally exhibit lower WF. In oxides/perovskites, materials with barely filled d-bands tend to show lower WF values. For example, a density-functional theory (DFT) screening of perovskite oxides found that those with barely filled d-bands and AO-terminated surfaces achieved predicted WF ~0.9–1.5 eV [18].
• Surface termination, morphology, and defects: Crystallographic termination [27], surface relaxation/contamination and reconstruction, atomic steps and facets, roughness [28,29], as well as defect density and adsorbates, influence both local potential and electronic states at the surface, altering WF [30,31,32].
• Dimensional confinement and nanostructuring: For two-dimensional (2D) materials and nanostructures, quantum confinement, altered screening, and increased surface-to-volume ratio can reduce WF with respect to bulk values. Alkali-metal-adsorbed transition metal dichalcogenides (TMDs) were predicted to reach WF < 1 eV under idealized conditions [33]. In particular, geometry-induced quantum effects arising from surface nanostructuring provide an additional, fundamentally distinct route to WF and Fermi-level engineering. Periodic nanoscale features—such as nanogratings, nanopillars, and corrugated surfaces—can modify the electronic density of states through quantum confinement and boundary-condition effects, leading to an effective redistribution of carriers and a shift in the Fermi level, a mechanism often referred to as geometry-induced doping (G-doping) [34]. Unlike conventional chemical doping, G-doping does not rely on impurity incorporation but instead originates from the spatial modulation of the electronic wavefunctions imposed by nanoscale geometry (Figure 3) [35]. These effects can be coupled with enhanced sensitivity to adsorbates and surface dipoles, amplifying WF tuning compared to bulk counterparts. As a result, quantum confinement acts as a synergistic mechanism that complements dipole engineering and Fermi-level tuning in the design of LWF nanomaterials [36], which is of particular interest in nanostructured and vacuum microelectronic devices.

Figure 2. Schematic band diagrams illustrating equilibrium and non-equilibrium concepts relevant to WF and electron emission in semiconductors. (a) Idealized equilibrium band alignment for a bare surface under dark conditions, showing vacuum level (E_vac), Fermi level (E_F), electron affinity (EA), conduction band minimum (CBM), valence band maximum (VBM), and equilibrium WF. (b) Same conditions as (a), but in the case of negative electron affinity (NEA). (c) Equilibrium band alignment for a surface exhibiting band bending (BB) due to surface states (indicated by the small blue curves in the sketch) or interface effects, illustrating the distinction between bulk electronic structure and surface energetics. BB can upshift (as in the scheme) or downshift the WF, thus to consider a measured effective WF (i.e., true emission barrier). (d) Non-equilibrium conditions under illumination, where surface photovoltage (SPV) partially compensates BB and shifts near-surface energy levels without altering the intrinsic bulk electronic structure.

Figure 2. Schematic band diagrams illustrating equilibrium and non-equilibrium concepts relevant to WF and electron emission in semiconductors. (a) Idealized equilibrium band alignment for a bare surface under dark conditions, showing vacuum level (E_vac), Fermi level (E_F), electron affinity (EA), conduction band minimum (CBM), valence band maximum (VBM), and equilibrium WF. (b) Same conditions as (a), but in the case of negative electron affinity (NEA). (c) Equilibrium band alignment for a surface exhibiting band bending (BB) due to surface states (indicated by the small blue curves in the sketch) or interface effects, illustrating the distinction between bulk electronic structure and surface energetics. BB can upshift (as in the scheme) or downshift the WF, thus to consider a measured effective WF (i.e., true emission barrier). (d) Non-equilibrium conditions under illumination, where surface photovoltage (SPV) partially compensates BB and shifts near-surface energy levels without altering the intrinsic bulk electronic structure.

The interplay of bulk electronic structure (i.e., Fermi level), surface dipole/potential step, and surface chemistry/morphology thus defines the challenge for designing materials with ultra-low WF. Understanding these mechanisms is crucial for the rational design of LWF materials and interface engineering in devices. Computationally, slab DFT models with sufficient vacuum spacing, dipole corrections, correct surface terminations, and well-converged calculations are standard to predict WF. High-throughput frameworks and machine learning (ML) models now help scan large material spaces by using specific descriptors to approximate WF trends. However, bridging theory with experimental applications remains challenging: real surfaces have adsorbates, patches with varying local WF, contamination, morphological variation, and measurement-geometry issues. For instance, the recent review by Lin et al. [37] emphasizes heterogeneous surface patch fields and their effect on measured WF values.

Figure 3. Geometry-induced doping (G-doping) via surface nanostructuring. (a) Schematic illustration of periodic surface nanostructuring, such as nanogratings or nanopatterns, which modifies the electronic boundary conditions without introducing chemical impurities. (b) Corresponding density-of-states redistribution induced by geometric confinement (indicated by the arrows from valence band to the conduction band), leading to an effective upward shift in the Fermi level in intrinsic, p-type, and n-type semiconductors. This geometry-induced modification of the electronic structure enables Fermi-level tuning and WF engineering through purely structural means rather than conventional chemical doping. Reproduced from Ref. [35]. Copyright: open access, credit to the original authors.

Figure 3. Geometry-induced doping (G-doping) via surface nanostructuring. (a) Schematic illustration of periodic surface nanostructuring, such as nanogratings or nanopatterns, which modifies the electronic boundary conditions without introducing chemical impurities. (b) Corresponding density-of-states redistribution induced by geometric confinement (indicated by the arrows from valence band to the conduction band), leading to an effective upward shift in the Fermi level in intrinsic, p-type, and n-type semiconductors. This geometry-induced modification of the electronic structure enables Fermi-level tuning and WF engineering through purely structural means rather than conventional chemical doping. Reproduced from Ref. [35]. Copyright: open access, credit to the original authors.

3. Strategies for Lowering the WF

Reducing the WF of a material can be achieved through several complementary engineering strategies. These aim to modify either the Fermi level (raising it towards the conduction band minimum) or the vacuum surface potential (lowering it) and must be balanced with stability and conductivity considerations.

3.1. Surface Dipole Engineering

One of the most direct approaches is to induce an outward-pointing dipole at the surface that lowers the vacuum level relative to the Fermi level. Adsorbing alkali metals (Cs, K) or their compounds on metal/oxide surfaces has been a classical method, achieving WF reductions of ~2–3 eV. However, these coatings tend to be reactive and unstable in air. More recently, molecular self-assembled monolayers (SAMs) and organic dipolar layers have been explored as more stable alternatives, enabling WF reductions of ~0.5–1 eV on transparent conductive oxides for optoelectronic applications [38]. In contrast to alkali-metal adsorption, atomically thin layers, such as graphene or h-BN, modify the work function through interface dipoles arising from charge redistribution, Pauli pushback effects, and weak van der Waals interactions rather than purely ionic bonding; depending on the balance between these contributions, the net effect can result in either an increase or a decrease in the WF [39,40]. Importantly, these dipoles can be modulated by layer thickness, stacking configuration, strain, and electrostatic environment, enabling continuous tuning of the WF [41], while simultaneously acting as protective barriers against oxidation and contamination. Experimental demonstrations on h-BN/LaB₆ heterostructures have shown that monolayer coatings can reduce the WF of ~0.4 eV while significantly enhancing chemical stability (Figure 4) [23], highlighting the dual role of 2D materials as both dipole-engineering and passivation layers. Indeed, it is important to clarify that the role of 2D overlayers in LWF systems requires careful qualification. In general, adding an overlayer modifies the surface dipole and, therefore, the WF; a 2D layer does not simply “transmit” the underlying WF unchanged. However, under specific conditions, atomically thin overlayers can maintain a low effective emission barrier while improving chemical robustness. This can occur when the overlayer is weakly coupled to the substrate and modifies the vacuum level through interfacial effects without fully suppressing electron emission from low-barrier regions of the underlying surface. In such cases, the measured WF reflects an average over heterogeneous surface regions, where locally low-barrier areas dominate emission despite partial coverage.

Importantly, the stabilizing effect of 2D overlayers does not rely solely on intentionally patchy coverage. Even continuous monolayers can enhance stability by acting as diffusion barriers against oxidation and contamination, while still allowing for tunneling or thermally assisted electron transmission through atomically thin regions. The relevant figure of merit in these systems is therefore not the equilibrium work function of the capped surface alone, but the device-effective emission barrier, which can remain low if interfacial dipoles and weak coupling preserve favorable band alignment. Clarifying this distinction reconciles the apparent contradiction between work-function modification and enhanced operational stability introduced by 2D overlayers.

3.2. Doping and Defect Engineering

Another route involves modifying the bulk electronic structure via doping (n-type) or the introduction of defects (oxygen vacancies, interstitials) to raise the Fermi level or increase conduction [42]. In conductive oxides and perovskites, introducing aliovalent dopants (e.g., Nb, Mo) or oxygen deficiency shifts the Fermi level upward and reduces WF [43]. In diamond-based systems, nitrogen incorporation does not intrinsically reduce the WF in a direct manner. Nitrogen acts primarily as a deep donor in diamond, and its influence on electron emission is strongly mediated by surface chemistry and activation conditions. In particular, hydrogen termination is essential to induce NEA, while nitrogen doping mainly enhances bulk conductivity and electron supply to the surface. Consequently, low effective emission barriers in nitrogen-doped diamond arise from the combined effects of bulk donor states, surface termination, and band bending, rather than from nitrogen doping alone [44]. Yet, high doping or vacancy concentrations may impair mobility, introduce trap states, or compromise stability.

3.3. Nanostructuring and Dimensional Control

Nanostructured morphologies—nanorods, nanowires, nanosheets, and tip arrays—offer geometric enhancement of local electric fields and altered surface potentials, effectively lowering the emission barrier and apparent WF. Quantum confinement and altered electronic screening in 2D materials also allow for further WF tuning. Both theoretical and experimental studies have demonstrated that such nanostructuring can induce measurable Fermi-level shifts and WF changes in metals, semiconductors, metal–semiconductor interfaces, and p–n junctions [45]. These effects have been observed across a broad range of material systems and offer a complementary pathway for tuning electronic properties, while avoiding chemical disorder and dopant-related instability. Alkali-metal-adsorbed TMDs, predicted to have a WF < 1 eV in ideal conditions [33], are example systems, although air stability remains a challenge.

3.4. Interface and Heterostructure Engineering

Creating heterostructures that combine different materials allows for charge transfer, interfacial dipoles, and composite surfaces with tailored WF. For example, Cs/Cs-O absorbed graphene on semiconductors leverages graphene’s high conductivity and the low WF of alkali adsorbates, achieving a WF of ~1.25 eV [41,46]. In devices such as photocathodes or OLEDs, inserting low WF interlayers or dipole layers at contacts is now standard practice for WF tuning.

3.5. Synergistic Combinations and Trade-Offs

Since each individual strategy has advantages and limitations, modern research emphasizes combining methods, e.g., doping a conductive oxide, nanostructuring it, and capping with a 2D protective layer to achieve a WF of <2 eV with improved stability [19]. ML tools help navigate the multi-parameter design space (WF, stability, conductivity, and processability) and identify promising multi-functional materials [17].

4. Families of LWF Materials

Here, the major material classes that have been explored for achieving LWF are reported, by summarizing their merits, typical WF ranges, and key challenges. Table 1 shows the most important results (both theoretical and experimental) present in the literature for LWF materials owing to the classes reported in the next sub-sections.

4.1. Alkali Metals and Alkali-Based Compounds

Alkali metals (Cs, K, Rb, Na, and Li) possess some of the lowest intrinsic work functions among all the elements due to their single loosely bound valence electron and highly polarizable surfaces. Cesium and potassium, in particular, have been foundational in vacuum electronics and photocathode technology for nearly a century.

Cesium exhibits an exceptionally low WF (~1.8–2 eV for clean surfaces [47]), which can be reduced further through surface reconstructions or co-adsorption with oxygen or halides. Potassium demonstrates similar LWF values (~2.2 eV) [48], but both Cs and K suffer from extreme chemical reactivity and limited thermal stability. Clean alkali surfaces oxidize within seconds in ambient conditions, limiting practical deployment to ultra-high-vacuum environments or encapsulated architectures.

Alkali activation layers, such as Cs–O, Cs–K, and Cs–K-Rb [49] films, are widely used to reduce the effective WF of substrates like refractory metals or semiconductor photocathodes.

Cs deposition on 2D materials (e.g., MoS₂, graphene, h-BN) has been shown to reduce their WF by several eV, depending on coverage and adsorption geometry, opening new pathways for low-energy electron emission and interfacial electronic engineering. Alkali metal incorporation into host lattices (e.g., K-doped graphene, Cs-intercalated graphite, alkali-doped fullerides, K-doped nitrides) enables more stable LWF surfaces while mitigating reactivity. These materials exhibit WF reductions of 0.5–2.0 eV relative to their pristine counterparts and often maintain stability under mild vacuum or encapsulation.

4.2. Borides and Nitrides

Materials, such as LaB₆, have been longstanding cathode materials (WF ≈ 2.3 eV) due to their high thermal stability and metallic conduction. Thin films of LaB₆ on silicon or tungsten substrates presented higher WF values, due to their non-stoichiometric ratio or oxygen surface contaminations [50,51]. Recent studies have explored coatings (e.g., monolayer h-BN) and variations (rare-earth borides and alloyed heaxaborides) to reduce WF further [52]. Experimentally, the presence of oxygen in thin films of borides and nitrides was found to form non-stoichiometric compounds, contrasting the achievements of LWF values [53].

Transition metal nitrides have also gained interest: for instance, a recent study shows that WF below a threshold value reduces the reducibility of TMNs in a hydrogen environment, by linking WF to chemical stability [54].

4.3. Barium- and Scandium-Based Oxides

Barium- and scandium-based oxides represent two of the most historically important classes of LWF materials. Ba–O systems have been widely used in thermionic cathodes due to their ability to form surface dipoles that dramatically reduce the effective work function when Ba adatoms segregate to the surface. BaO, deposited on porous W [55] or other metallic substrates [56], achieves effective work functions down to the 2.0 eV range under operating conditions because of the formation of sub-stoichiometric Ba–O phases and Ba adsorption layers that enhance electron donation. The extremely high vapor pressure of metallic Ba, combined with its strong chemical reactivity, results in continuous surface renewal, which helps maintain LWF but also introduces issues related to chemical instability, evaporation losses, and lifetime limitations. In contrast, scandium oxide (Sc₂O₃) has attracted considerable attention as a more stable alternative for LWF systems [57]. Sc₂O₃ possesses a wide band gap (~6 eV) and becomes a good electron emitter when lightly doped or when oxygen vacancies are incorporated, which introduce donor states below the conduction band [58]. Reported effective work functions for Sc₂O₃-based cathodes range from 1.2 to 1.6 eV, depending on operating temperature, vacancy concentration, and co-adsorbates such as Ba, Ca, or activated oxygen species [59]. Sc₂O₃ layers also exhibit strong thermochemical stability, high melting point, and lower volatility than BaO, making them suitable for high-temperature or long-lifetime applications. Furthermore, Sc₂O₃ surfaces can synergistically interact with Ba-containing phases in mixed Ba–Sc–O cathodes, where Ba adsorption on Sc₂O₃ has been shown to further reduce the WF by enhancing surface dipoles while maintaining better chemical endurance than pure BaO layers. Recent comprehensive reviews by Beck and co-workers [60,61] have clarified that emission enhancement in Sc-containing cathodes does not arise from a single universal mechanism, but rather from a complex interplay between Ba surface coverage, Sc-containing phases, oxygen vacancy chemistry, and local microstructural heterogeneity. These studies emphasize that both BaO-based and scandate cathodes can achieve similarly effective LWF values under optimized conditions, while exhibiting markedly different stability and reproducibility characteristics depending on fabrication route and operating environment. In particular, scandate cathodes can display WF values comparable to, or even lower than, traditional Ba-based systems, but with significant variability in emission stability and temporal evolution. These findings underscore that lifetime improvements are highly sensitive to microstructure, Ba supply dynamics, and oxygen activity, rather than being an intrinsic property of scandium-containing oxides alone. Finally, it is possible to state that Ba–O and Sc₂O₃ oxides represent two complementary strategies for LWF design [62]: their combined use in modern composite oxide cathodes highlights how defect engineering, surface adsorption, and controlled stoichiometry can be exploited to tailor electron emission performance.

4.4. Conductive Oxides, Perovskites, Polymeric and Organic/Hybrid Electrodes

Conductive oxides, metallic perovskites, and polymer-based systems constitute an increasingly important class of LWF materials, particularly in contexts where chemical stability, scalability, and compatibility with large-area processing are critical. In metallic perovskite oxides and related correlated systems, the WF is not a fixed bulk property but emerges from a delicate balance between surface termination, oxygen stoichiometry, and electronic correlations. Both first-principles calculations and experiments have demonstrated that AO-terminated or oxygen-deficient surfaces can exhibit effective work functions approaching or falling below 2 eV, while maintaining metallic conductivity and structural robustness. DFT screening of ~2900 compounds found seven candidates with a predicted WF as low as ~0.9–1.5 eV. Experimental confirmation has been emerging: SrVO₃ showed evolution during cathode activation with effective WF ~2.3 eV under thermionic emission [63]. These features make conductive oxides especially attractive as electron emitters and contact layers operating under harsh thermal or chemical conditions.

Beyond inorganic oxides, polymeric and organic–inorganic hybrid materials offer a fundamentally different yet complementary route to WF reduction. In these systems, low effective WF values are achieved not through intrinsic electronic structure alone, but via strong interfacial dipoles formed by molecular orientation, charge transfer, or ionic functional groups. Conjugated polymers, polyelectrolytes, and molecular interlayers have been shown to reduce the effective work function of metal or oxide electrodes by more than 1 eV, enabling efficient electron injection in optoelectronic and vacuum microelectronic devices, such as organic solar cells and LEDs. For example, insertion of a lacunary polyoxometalate interlayer on TiO₂ reduced WF and boosted solar cell efficiency by ~25–33% [64]. Importantly, polymer-based LWF layers combine moderate WF values with solution processability, mechanical flexibility, and tunable interfacial chemistry, attributes that are difficult to achieve simultaneously in purely inorganic systems.

From a materials-design perspective, conductive oxides and polymeric systems illustrate a shift from the pursuit of extreme WF minima toward engineered interfaces that balance electronic performance with environmental stability and processability. Their inclusion broadens the LWF landscape beyond traditional cathode materials and highlights how interfacial control, rather than bulk composition alone, can dominate WF engineering in practical devices.

4.5. Carbon-Based Materials

Carbon-based materials—including diamond, graphene, carbon nanotubes, nanodiamond, and carbide derivatives—constitute a unique class with high stability, chemical inertness, and broad tunability of work function via surface termination, doping, and structural engineering.

Hydrogen-terminated diamond (H-diamond) is one of the most extraordinary LWF materials known. Due to the achievement of strong NEA conditions, H-terminated diamond can exhibit effective WF values as low as 0.9–1.3 eV [65,66], enabling exceptionally low-threshold field emission. Advantages are the extreme chemical and mechanical stability, the compatibility with high-voltage field emitters, as well as the potential for long-lifetime cold cathodes. However, achieving reproducible NEA requires precise control of surface quality, hydrogen coverage, and defect density [67]. Oxygen termination, in contrast, increases WF (> 4 eV), showing the huge impact of surface chemistry.

Carbon nanotubes (CNTs), nanodiamond, and amorphous carbon films exhibit strong field enhancement effects due to high aspect ratios, enabling effective low-threshold field emission even with moderate WF (~2.5–3.5 eV). Surface functionalization with nitrogen, hydrogen [68], or Ba-based species [69] can reduce WF by 0.5–1.5 eV and improve emission stability. CNT cold cathodes are among the most advanced carbon-based emitters, with operating fields as low as 1–2 V/µm, thanks to synergistic effects of WF reduction and geometric field enhancement.

4.6. Two-Dimensional Materials and Hybrids

Pristine graphene has a WF of ~4.5–4.6 eV [70], but it can be dramatically modified through alkali adsorption (with coverage of Cs, the WF was down to ~1.25 eV, as shown in Figure 5, where the WF drops to 1.05 eV when an electric field is applied [41]). Additionally, graphene doped with alkali metals has shown WF reductions of 1.5–2.0 eV [71], with improved stability when encapsulated with h-BN or atomic layer deposition (ALD) coatings. Due to its high conductivity and atomic thickness, graphene can also be an excellent platform for WF modulation in layered heterostructures.

Two-dimensional materials, such as TMDs with alkali adsorption/termination engineering, show potential for ultra-low WF (even sub−1 eV in theory). A systematic DFT study on MXenes reported tunable WF via termination species (O, NH, F, Br) and found trends linking termination electronegativity and metal-termination distance with WF [72]. A record-LWF of 0.70 eV by inducing a surface photovoltage in an n-type GaAs with a Cs/O₂ coating was experimentally reported [73]. Yet, realizing stable LWF 2D surfaces in operating conditions remains a challenge.

To conclude, the landscape of LWF materials spans high-performance but fragile systems (e.g., alkali-based systems), stable but moderate-WF borides, nitrides, and oxides, highly tunable carbon materials, ultra-scalable polymers, and theoretically promising but experimentally challenging perovskites and 2D materials. The combination of surface dipoles, electronic-structure tuning, adsorbate engineering, and dimensionality control defines the modern toolkit for achieving LWF values below 2 eV with high stability.

Table 1. Summary of the main theoretical and experimental results reported in the literature for LWF materials. UHV indicates ultra-high vacuum conditions. N/A indicates that the corresponding information was not reported or could not be reliably extracted from the original literature sources.

Material	Work Function (eV)	Uncertainty	Theoretical/Experimental	Method/Experimental Details/Notes
K-adsorbed WTe2 (2D)	0.7		Theoretical [33]
Cs/O2 on n-GaAs	0.7	0.1 eV	Experimental [73]	Photoemission low-energy cutoff (LEC), UHV under illumination (SPV). WF = 1.06 ± 0.1 eV, if not illuminated.
P-doped H-diamond/Si	0.9	N/A	Experimental [66]	Thermionic method (UHV, 650–1000 K)
BaZr0.375Ta0.5Fe0.125O3	0.93		Theoretical [74]	AO-terminated
BaMoO3	1.06		Theoretical [74]	AO-terminated
Ba0.25Sc0.25O on W (001)	1.16		Theoretical [75]
Cs/O on graphene	1.25	0.08 eV	Experimental [41]	Photoemission low-energy cutoff (LEC). WF = 1.32 eV measured with KFPM at ambient conditions. If back-gated, WF decreases to 1.01 ± 0.05 eV.
N-doped H-diamond/Re	1.34	N/A	Experimental [76]	Thermionic method (UHV, 525–750 K)
CsScCl3	1.42		Theoretical [17]	Termination: (100)-Sc, (100)-Cs-Cl
SrN2	1.59		Theoretical [17]	Termination: (110)-Sr
BaSi2	1.68		Theoretical [17]	Termination: (100)-Ba, (100)-Si
CsI/W (110)	1.69		Theoretical [77]
La2O3−x (hexagonal)	1.8		Theoretical [78]
K-Cs-Rb	≈1.8	< 1%	Experimental [49]	Fowler photoelectric method, UHV, 90–450 K
La0.25Ba0.75B6 (001)	1.84		Theoretical [52]
Ba0.5O/Hf (1012)	1.88		Theoretical [56]
SrMoO3	1.93		Theoretical [74]	AO-terminated
BaF2/GaAs	2.1	N/A	Experimental [79]	UPS (UHV, cutoff energy by applying a series of negative bias voltages)
HfN (001)	2.16		Theoretical [52]
Ca/ZnO (001)	2.25		Theoretical [80]
SrVO3 (polycrystalline)	≈2.3	0.1 eV	Experimental [81]	Thermionic method (800–1400 °C)
h-BN/LaB6	2.35	N/A	Experimental [23]	Scanning Tunneling Microscopy (UHV, 77 K)
Mo2C(NH)2	2.4		Theoretical [72]	NH-terminated
p-Pyrrd–Phen on ITO/ZnO	2.43	N/A	Experimental [82]	UPS (UHV)
Ce0.25La0.75B6 (single-crystal, 001 termination)	2.61	N/A	Experimental [21]	Field-assisted thermionic emission (UHV, 1673–1873 K)

Table 1. Summary of the main theoretical and experimental results reported in the literature for LWF materials. UHV indicates ultra-high vacuum conditions. N/A indicates that the corresponding information was not reported or could not be reliably extracted from the original literature sources.

Material	Work Function (eV)	Uncertainty	Theoretical/Experimental	Method/Experimental Details/Notes
K-adsorbed WTe2 (2D)	0.7		Theoretical [33]
Cs/O2 on n-GaAs	0.7	0.1 eV	Experimental [73]	Photoemission low-energy cutoff (LEC), UHV under illumination (SPV). WF = 1.06 ± 0.1 eV, if not illuminated.
P-doped H-diamond/Si	0.9	N/A	Experimental [66]	Thermionic method (UHV, 650–1000 K)
BaZr0.375Ta0.5Fe0.125O3	0.93		Theoretical [74]	AO-terminated
BaMoO3	1.06		Theoretical [74]	AO-terminated
Ba0.25Sc0.25O on W (001)	1.16		Theoretical [75]
Cs/O on graphene	1.25	0.08 eV	Experimental [41]	Photoemission low-energy cutoff (LEC). WF = 1.32 eV measured with KFPM at ambient conditions. If back-gated, WF decreases to 1.01 ± 0.05 eV.
N-doped H-diamond/Re	1.34	N/A	Experimental [76]	Thermionic method (UHV, 525–750 K)
CsScCl3	1.42		Theoretical [17]	Termination: (100)-Sc, (100)-Cs-Cl
SrN2	1.59		Theoretical [17]	Termination: (110)-Sr
BaSi2	1.68		Theoretical [17]	Termination: (100)-Ba, (100)-Si
CsI/W (110)	1.69		Theoretical [77]
La2O3−x (hexagonal)	1.8		Theoretical [78]
K-Cs-Rb	≈1.8	< 1%	Experimental [49]	Fowler photoelectric method, UHV, 90–450 K
La0.25Ba0.75B6 (001)	1.84		Theoretical [52]
Ba0.5O/Hf (1012)	1.88		Theoretical [56]
SrMoO3	1.93		Theoretical [74]	AO-terminated
BaF2/GaAs	2.1	N/A	Experimental [79]	UPS (UHV, cutoff energy by applying a series of negative bias voltages)
HfN (001)	2.16		Theoretical [52]
Ca/ZnO (001)	2.25		Theoretical [80]
SrVO3 (polycrystalline)	≈2.3	0.1 eV	Experimental [81]	Thermionic method (800–1400 °C)
h-BN/LaB6	2.35	N/A	Experimental [23]	Scanning Tunneling Microscopy (UHV, 77 K)
Mo2C(NH)2	2.4		Theoretical [72]	NH-terminated
p-Pyrrd–Phen on ITO/ZnO	2.43	N/A	Experimental [82]	UPS (UHV)
Ce0.25La0.75B6 (single-crystal, 001 termination)	2.61	N/A	Experimental [21]	Field-assisted thermionic emission (UHV, 1673–1873 K)

5. Measurement and Computational Techniques

The accurate determination of WF and its reliable prediction are both central challenges in the field of LWF materials design. From the experimental side, commonly used techniques include Ultraviolet Photoelectron Spectroscopy (UPS), Kelvin Probe Force Microscopy (KPFM), field or thermionic emission measurements, and, in some cases, angle-resolved photoemission (ARPES) or photoemission electron microscopy (PEEM). Figure 6 summarizes the features of the main techniques. On the computational side, slab DFT calculations remain standard for predicting WF, with growing contributions from high-throughput screening and ML-assisted models.

• Experimental methods: UPS can measure the difference between the vacuum level and the Fermi energy by analyzing the cut-off of emitted photoelectrons under ultraviolet excitation [83]. However, Helander et al. note important pitfalls [84], in that UPS typically measures the minimum WF (i.e., the lowest WF patch on an inhomogeneous surface) rather than an average, and measurement geometry (sample-detector alignment) and contamination critically influence the result. KPFM methods measure contact potential difference between a reference tip and the sample surface, enabling the mapping of WF variation across surfaces and operation in ambient or controlled atmospheres [85]. Nonetheless, tip WF drift, surface contamination, and stray fields must be carefully calibrated. Recently, an in-depth treatment of measurement artifacts, patch fields, and electric-field effects on measured WF values has been reported [37]. Field/thermionic emission experiments infer an effective WF from the temperature- or field-dependent emission current [86]; these methods reflect device-level performance but are influenced by surface morphology, local fields, and non-idealities. Based on these considerations, it is important to make a note regarding the experimental methods. Figure 7 provides a representative example of best practices in UPS-based WF measurements on semiconducting oxides. In particular, the application of a controlled bias voltage during cutoff acquisition minimizes spurious zero-field effects and charging-related distortions, enabling a more reliable determination of the equilibrium work function. This approach is consistent with established methodologies developed to mitigate measurement artifacts in complex semiconductor and dielectric stacks, as discussed in detail in the work of Martinez et al. [87]. Finally, it is important to underline that, when comparing reported WF values, best practices should include clearly specifying surface preparation, measurement environment (ultra-high vacuum vs. ambient), temperature, and uncertainty, as well as distinguishing between equilibrium WF and effective WF or illumination-assisted emission barriers. Such reporting is essential to ensure meaningful comparison across materials and techniques.
• Computational methods: DFT slab calculations model surface terminations and compute the potential drop from the slab Fermi level to the vacuum region, yielding theoretical WF [88]. Screening studies correlate bulk descriptors (e.g., d-band filling, oxygen p-band center) with predicted low WF values. A recent review summarizes workflows, errors, and correlation to the experiment [89]. A key challenge is modeling realistic surfaces: adsorbates, surface reconstructions, contamination, finite temperature effects, and polycrystalline facets often shift WF with respect to ideal slabs. In recent years, machine learning (ML) and data-driven computational approaches have increasingly complemented traditional DFT calculations in the study of WF and materials engineering [90]. By training specific models on extensive high-throughput DFT datasets, ML frameworks can rapidly predict WF values across vast chemical spaces, identify hidden structure–property correlations, and guide the discovery of unconventional LWF materials that may be overlooked by classical intuition. Beyond accelerating high-throughput calculations, recent ML-driven screening studies have begun to identify previously unexplored candidate LWF materials and surfaces. In particular, Schindler et al. [17] combined large-scale DFT databases with supervised ML classifiers to screen thousands of surface terminations, uncovering stable material–surface combinations exhibiting predicted WF values below 1.5–2.0 eV that had not been previously highlighted in the literature. Importantly, this approach explicitly incorporated thermodynamic stability and surface realism as selection criteria, rather than focusing solely on idealized electronic structure. Similar ML-assisted workflows applied to perovskite materials [74] have revealed non-intuitive composition–termination relationships governing WF reduction, demonstrating that data-driven models can guide the discovery of viable LWF candidates beyond manual DFT exploration. While experimental validation still remains limited, these studies represent concrete examples in which ML screening has gone beyond acceleration and has actively proposed new LWF materials and surface configurations for further investigation.

Figure 6. Comparison of experimental approaches for work function determination. The schematic summarizes the main characteristics of the most commonly used techniques for WF measurement, highlighting their operating principles, inherent advantages, and limitations.

Figure 6. Comparison of experimental approaches for work function determination. The schematic summarizes the main characteristics of the most commonly used techniques for WF measurement, highlighting their operating principles, inherent advantages, and limitations.

Generally, the most difficult aspect is bridging experiment and theory. Discrepancies of ~0.3–0.8 eV (or even more) between predicted and measured WF are common, due to patch fields, measurement inhomogeneity, WF distributions, and unaccounted contaminations in the surface chemistry. When surface structure and chemistry are well controlled, discrepancies between computed and experimental WF can be reduced to ≈0.2–0.3 eV, as demonstrated for carefully prepared surfaces, like reported for SrTiO₃ [91,92]. Therefore, the larger deviations frequently reported in the literature mainly arise from heterogeneous surface patch fields, uncontrolled adsorbates, and measurement-specific artifacts rather than intrinsic limitations of electronic-structure methods. This distinction is critical to avoid overestimating the uncertainty of first-principles predictions when evaluating realistic LWF candidates. In particular, heterogeneous surface patch fields (micro-areas with varying local WF) can significantly affect emission and measurement. A unified approach—reporting surface preparation, measurement conditions, and combining complementary techniques—is lacking but strongly recommended to enhance comparability across studies.

In summary, robust WF evaluation requires the following: (i) clear documentation of surface preparation/conditions, (ii) use of complementary measurement techniques, (iii) comparison with theoretical predictions, and (iv) recognition of patch variability and field effects. These practices will enable more reliable advancement of LWF materials.

6. Stability, Processability, and Scalability

While significant attention has been devoted to achieving record-low WF values, it is important to emphasize that the technologically most relevant regime is often not the absolute minimum, but rather the sub-2 eV range combined with stability and reproducibility. Of course, a WF below 2 eV enables qualitatively new operating windows for electron-emission-based devices, allowing for efficient emission at substantially reduced temperatures or electric fields. In thermionic and photon-enhanced thermionic systems, this regime relaxes constraints on thermal management, mitigates materials degradation, and broadens compatibility with scalable substrates. Similarly, for photocathodes and vacuum microelectronic devices, sub-2 eV work functions reduce stringent vacuum requirements and enhance charge injection efficiency without relying on highly reactive or short-lived activation layers. From a materials-design perspective, targeting this range represents a pragmatic balance between performance gains and surface realism, shifting the focus from extreme but fragile ultra-low WF configurations toward robust, manufacturable solutions that can operate under realistic conditions.

Therefore, while achieving a LWF is a necessary milestone, translating such materials into real-world devices demands equal attention to stability, processability, and scalability—often recognized as the critical bottlenecks in the field of LWF materials. Three major aspects define this challenge:

• Chemical and environmental stability: Many ultra-low WF surfaces are highly reactive: they oxidize, adsorb ambient species (e.g., O₂, H₂O), or restructure when exposed to air or multiple thermal/field cycles. For example, nanoscale emitters of LaB₆ show extremely stable emission only when their surface is covered by lanthanum oxides (LaO, La₂O_3-x) [78], which retain an LWF while improving chemical robustness. Another strategy is forming protective 2D overlayers (e.g., h-BN) on LWF cores, which preserve the LWF values while shielding surfaces from contamination or oxidation. However, long-term ambient tests (months to years, under cycling) remain relatively rare in the literature.
• Processability and compatibility with device fabrication: For deployment in devices (i.e., large-area cathodes, printable electronics, and flexible substrates), LWF materials must be compatible with cost-effective deposition techniques (like solution process, thermal evaporation, sputtering, and roll-to-roll), patterning, and integration with other layers. Some polymer-based LWF electrodes (solution-processed) demonstrate ambient stability and ease of fabrication, but they often do not reach WF values as low as the best inorganic systems, and they can be used in high-temperature applications. The trade-off between performance (WF reduction) and manufacturability must be navigated.
• Scalability and uniformity: Scaling from small-area laboratory samples to device-scale (cm² or more) introduces new challenges, such as uniformity of surface composition, maintenance of LWF over a large area, defects’ control, reproducibility, and system cost. Many screening studies identify candidate materials with excellent LWF in ideal conditions, but few address film growth, deposition yield, process tolerances, or long-term device-integration stability. For instance, while perovskite oxides with promising LWF are intellectually exciting, their thin-film growth, surface termination control, and large-area reproducibility remain open issues.

In practice, the ideal LWF material must fulfill a “triple constraint”: (i) sufficiently low value of work function needed for the application, (ii) robust stability under relevant environmental/operational conditions, and (iii) scalable fabrication and integration into device architectures. Research that addresses all three simultaneously remains comparatively sparse, but these constraints are essential if LWF materials need to pass from laboratory curiosities to industrially viable components.

7. Emerging Applications

LWF materials underpin a wide range of technologies that rely on efficient electron emission, injection, or extraction. Their applications span from classical vacuum electronics to next-generation optoelectronic, energy, and quantum devices. The following subsections summarize major and emerging application areas, highlighting performance metrics and recent developments.

7.1. Vacuum and Field Emission Devices

Historically, LWF materials have been essential in vacuum electronics—thermionic converters, cathode ray tubes, and microwave amplifiers. Modern interest has shifted toward micro- and nano-scale electron sources, including cold cathodes for flat-panel displays, X-ray tubes, and miniature electron guns.

Nanostructured LaB₆ [93], H-terminated diamond, and carbon nanotubes are key candidates: they offer WF < 3 eV and exceptional field emission stability [94]. For instance, LaB₆ nanoneedles fabricated by focused ion beam maintain emission stability over time under high current density, outperforming traditional tungsten emitters [95]. Additionally, MXene-based cold cathodes are also gaining attention due to their metallic conductivity and tunable WF through surface termination engineering [96].

7.2. Photocathodes and Photoemission

High-performance photocathodes for free-electron lasers (FELs) and accelerators require LWF with high quantum efficiency and fast response time, as well as ambient or vacuum operational durability.

LWF photocathodes (e.g., Cs, CsF, Cs–K–Sb, Cs₂Te, and GaAs activated by Cs/O) [97,98] are state-of-the-art but suffer from poor air stability. Hybrid architectures (e.g., Cs-coated metals, oxide/metal couples, 2D protective caps) aim to deliver low effective WF and long lifetime (e.g., under photon bombardment) [99,100]. Recent research explores perovskite and 2D semiconductor photocathodes that combine low WF with improved tolerance to ambient exposure [101]. These approaches promise next-generation photocathodes that combine high brightness, fast response, and long lifetime.

7.3. Optoelectronic and Energy Devices

Work function engineering is a cornerstone in organic and perovskite photovoltaics (OPV, PSC) [102] and light-emitting diodes (OLEDs, PeLEDs), where interfacial energy alignment governs charge extraction and injection.

In solar cells, inserting an LWF electron transport layer (e.g., doped ZnO, TiO₂ modified by SAMs or polymers) reduces interfacial barriers and boosts power conversion efficiency. Similarly, in OLEDs, cathodes with optimized WF enable efficient electron injection, lowering turn-on voltages and improving luminance stability. The development of printable, air-stable low-WF materials has therefore become critical for large-area flexible electronics and roll-to-roll device manufacturing.

7.4. Thermionic and Energy Conversion Devices

LWF materials are also fundamental for TECs, photon-enhanced thermionic emission converters [103,104,105], and hybrid thermionic-photovoltaic converters [106,107], where they allow for electron emission at lower temperatures, improving conversion efficiency. Moreover, WF is a key factor, for the optimization of both the electrodes (cathode and anode), and the determination ofthe output voltage of the converter (Figure 8). For these kinds of devices, a LWF is not sufficient for achieving outstanding performance, but it needs to be correlated with other materials’ properties (e.g., Richardson constant, electrical conductivity, thermal stability, device coupling) to obtain high-efficiency converters [11]. Coupled with solar concentrators [108] or waste-heat harvesting systems, these devices could enable new forms of clean energy generation.

In addition to the already cited LaB₆ and H-terminated diamond, cesiated semiconductors, most notably GaAs, are playing an important and continuing role in TECs and photon-enhanced thermionic energy conversion [109]. When activated with Cs, and often co-activated with oxygen, GaAs surfaces can exhibit extremely low effective emission barriers due to strong surface dipoles and illumination-induced reinforcement of reduced EA conditions [73]. Also, Si and InP are promising materials on which Cs can be applied to form an efficient cathode material for photon-enhanced thermionic emission [110].

7.5. Quantum, Sensor, and Emerging Frontiers

In emerging quantum and sensor technologies, WF plays a subtler but equally critical role. WF alignment controls tunneling rates and contact resistance in 2D materials and quantum devices. Lowering the WF of metal oxides improves coupling in Josephson junctions and superconducting qubits by reducing interface barriers [111].

Similarly, in gas sensors and photoelectrochemical cells, WF tuning enables selective adsorption and catalytic activation [112]. Organic–inorganic hybrid photoelectrodes for water splitting have demonstrated enhanced emission stability through encapsulation with thin 2D layers (h-BN, MoS₂) as efficient heterostructures, maintaining performance and photostability over time [113]. A recent review reported enhanced sensing response using MXenes-based composited when the WF is reduced and matched [114], demonstrating the potential for multifunctional sensing and catalytic applications.

Table 2. Representative classes of LWF materials and their primary application domains.

Material Class	Representative Examples	Typical Applications
Alkali compounds	Cs, CsO2, K-Cs	Thermionic cathodes, photocathodes
Rare-earth borides	LaB6	High-temperature emission devices
Ba-based systems	BaO, Ba-Sc-O	Thermionic cathode, photocathodes
Perovskites	SrVO3, SrTiO3	Robust emitters and electronics
Carbon-based materials	Diamond, CNT	Vacuum electronics, energy conversion devices
2D heterostructures	Graphene, h-BN	Interface engineering, protective layers, and WF tuning
Hybrid/organic layers	Polymer interlayers	Charge injection, flexible electronics

provides a qualitative overview linking major material families discussed in this review to typical application areas, highlighting how different LWF concepts address distinct device requirements along several technological fields.

In the end, despite the recent advances, the path to commercialization remains complex. Challenges include maintaining LWF under real-world conditions, achieving large-area uniformity, and developing scalable synthesis methods. Interdisciplinary approaches—combining materials design, device physics, and ML—are increasingly necessary.

The integration of LWF materials into flexible, transparent, and hybrid systems (such as transparent cathodes, energy converters, wearable sensors, or hybrid optoelectronics) represents a frontier where performance, stability, and manufacturability must converge. As research accelerates, cross-field collaboration will be essential to transition these discoveries from the lab to industry.

8. Open Challenges and Future Directions

While major advances have been made in identifying LWF materials and engineering interfaces, several critical challenges remain before these materials can be deployed in real devices. A first and fundamental issue concerns the lack of standardized measurement protocols. Reported WF values often vary widely due to differences in surface preparation, measurement technique, atmosphere, temperature, and surface morphology. This variability complicates meaningful comparison across studies and obstructs the capability of finding real structure–property relationships. Establishing community-agreed protocols, including reference materials, reporting standards, and uncertainty estimations, would substantially improve reproducibility and accelerate materials screening. A second major challenge is the significant gap between theoretical predictions and experimental realizations. Most computational studies evaluate idealized slab surfaces under equilibrium conditions, whereas experimental surfaces are affected by several non-idealities, such as adsorbates, oxide layers, reconstructions, finite temperature, and spatial heterogeneity (i.e., patch fields). As a result, promising theoretical candidates often fail to experimentally reproduce the predicted ultra-low WF. Bridging this gap requires tightly coupled experimental–computational workflows, in which surface chemistry, termination, morphology, and measurement conditions are documented with sufficient fidelity to enable realistic modeling and direct validation. Moreover, environmental and operational stability represent other major bottlenecks. While WF values below 2 eV have been demonstrated in several material systems, maintaining such values over extended lifetimes (> 500 h) under device-relevant conditions remains exceedingly rare. Surface degradation driven by oxidation, contamination, diffusion, or phase instability continues to limit practical implementation. Future research should therefore prioritize strategies that combine intrinsic LWF with surface passivation, encapsulation, or dynamic reconstruction mechanisms, rather than focusing exclusively on the achievement of record-low values but under unstable conditions. Equally important is the challenge of scalability and manufacturability. Many candidate LWF materials rely on complex synthesis routes, requiring stringent vacuum environments, or offering small-area demonstrations that are difficult to translate to wafer-scale or large-area devices where uniformity is needed. Addressing this limitation will require greater efforts on the use of scalable and cost-effective deposition techniques, materials becoming compatible with existing manufacturing infrastructures or the creation of new dedicated platforms, and tolerance to possible processing-induced variations. Finally, particular attention must be dedicated to the production of multi-functional materials and interfaces that simultaneously satisfy LWF values, have adequate electrical conductivity, and posses long-term thermal and mechanical stability, with processing characterized by scalability and uniformity. Of course, navigating this inherently multi-objective design path exceeds the capabilities of intuition-driven exploration alone. In this context, ML and data-driven materials discovery represent the most promising route to identify non-obvious trade-offs and guide the co-optimization of electronic, chemical, and thermal surface properties. To address this, progress in LWF functional materials will depend on interdisciplinary collaboration spanning surface science, computational materials design, device physics, and manufacturing engineering, enabling the translation of laboratory-scale breakthroughs into robust and industrially relevant technologies.

9. Conclusions

While existing reviews provide detailed treatments of WF fundamentals and methodologies, this work focuses on the gap between predicted and experimentally realized LWF values under realistic surface and operating conditions. By emphasizing interfaces, stability, and scalability, the aim is to identify which materials concepts remain viable for practical implementation rather than idealized benchmarks alone.

This review has surveyed recent advances in the discovery, engineering, measurement, and application of LWF materials. Over the last decade, progress in high-throughput computational screening, ML-assisted materials discovery, and refined surface engineering has greatly expanded the palette of candidate systems. Families, such as perovskites, MXenes, borides, nitrides, Ba- and Sc-based systems, carbon-based materials, and alkali-modified compounds (both hybrid 2D or bulk materials), now provide realistic pathways toward WF values below 2 eV in controlled conditions, and in some cases approach or reach the 1 eV regime under special surface preparation and protection strategies.

While significant attention has been devoted to achieving record-low WF values, it is important to emphasize that the most technologically relevant regime is often not the absolute minimum, but rather the sub-2 eV range combined with stability and reproducibility. WFs below 2 eV enable qualitatively new operating windows for electron-emission-based devices, allowing for efficient emission at substantially reduced temperatures or electric fields. In thermionic and photon-enhanced thermionic systems, this regime relaxes constraints on thermal management, mitigates materials degradation, and broadens compatibility with scalable substrates. Similarly, for photocathodes and vacuum microelectronic devices, sub-2 eV work functions reduce stringent vacuum requirements and enhance charge injection efficiency without relying on highly reactive or short-lived activation layers. From a materials-design perspective, targeting this range represents a pragmatic balance between performance gains and surface realism, shifting the focus from extreme but fragile LWF configurations toward robust, manufacturable solutions that can operate under realistic conditions. In this context, alkali-metal coatings and alkali-activated systems achieve some of the lowest effective WF values, but they are highly reactive and require stringent vacuum conditions or encapsulation to preserve performance. On the other hand, chemically robust materials, such as LaB₆, nitrides, and H-terminated diamond, demonstrate superior longevity but typically need additional approaches (e.g., coatings, doping, nanostructuring) to approach the lowest WF thresholds. Therefore, strategies that combine an intrinsically LWF core with an ultrathin protective overlayer (for example, 2D caps or ALD oxides) are especially promising for reconciling low WF with ambient durability.

Bridging computational predictions and experimental realization remains essential. Predictive workflows must incorporate realistic surface chemistries (adsorbates, oxide shells), finite-temperature effects, and morphological heterogeneity, while experimental studies should adopt and report standardized surface preparation and WF measurement protocols (UPS, KPFM, and thermionic/field emission parameters). Integrated experiment–theory efforts, complemented by open databases for WF and surface properties, as well as an integrated multi-stage platform combining several processes in a single route, are lacking but mandatory to accelerate the identification of multi-property candidates that simultaneously balance WF, conductivity, stability, and manufacturability.

Looking forward, three directions are particularly promising: (i) heterostructure/interface engineering that pairs LWF cores with atomically thin protective caps to deliver both LWF and ambient stability; (ii) scalable deposition and patterning approaches (sputtering, thermal evaporation, solution processes) for promising families, such as perovskites, MXenes, and coated borides to enable device-scale implementation; and (iii) community adoption of standardized measurement protocols and open datasets to power robust ML models for multi-objective materials discovery. Success on these fronts would enable the translation of laboratory demonstrations into next-generation devices, such as electron sources, energy converters, and efficient opto-electronic interfaces for sensing, catalytic, or quantum applications.

In summary, the quest for LWF materials is both scientifically rich and technologically consequential. Continued interdisciplinary research across surface chemistry, computational materials science, device engineering, and manufacturing technology will be required to realize durable, scalable, and high-performance LWF materials for practical applications.