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Article

Reversible Tuning of Surface Properties of Graphene-like Material via Covalently Functionalized Hydrophobic Layer

by
Thi Mien Trung Huynh
1,
Duy Dien Nguyen
1,
Nhat Hieu Hoang
2 and
Thanh Hai Phan
2,*
1
Department of Chemistry, Faculty of Natural Sciences, Quy Nhon University, 170 An Duong Vuong, Quy Nhon 820000, Vietnam
2
Department of Physics and Materials Science, Faculty of Natural Sciences, Quy Nhon University, 170 An Duong Vuong, Quy Nhon 820000, Vietnam
*
Author to whom correspondence should be addressed.
Crystals 2023, 13(4), 635; https://doi.org/10.3390/cryst13040635
Submission received: 3 March 2023 / Revised: 19 March 2023 / Accepted: 2 April 2023 / Published: 7 April 2023
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Abstract

:
Nanoscale tuning of the surface properties of graphene-like materials is essential to optimize their application in electronic devices and protective technologies. The covalent modification method has recently been established as the most effective approach for tailoring the interface structure and properties, which are key aspects for fine-tuning the processability and performance of graphene-like materials. In this work, we demonstrate systematic exploration of the reversible covalent functionalization of a highly oriented pyrolytic graphite (HOPG) surface, a model system of multi-layered graphene, at the molecular scale. This is achieved using 3,5-trifluoromethyl benzenediazonum (3,5-TFD) and experimental investigations via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning tunneling microscopy (STM), and Raman spectroscopy. The degree of functionalization could be tuned by varying the concentration of 3,5-TFD dissolved in the grafting electrolyte. The covalently functionalized layer of 3,5-TFD was either locally degrafted by the STM tip or globally detracted upon thermal treatment, leaving the defect-free graphitic surfaces behind. Our findings open a new pathway for reversibly and robustly functionalizing graphene and other 2D materials for multiple uses in high-end applications.

1. Introduction

Graphene-like materials have been intensively studied due to their unique properties [1]. More importantly, the chemical functionalization of graphene-like surfaces that aims to tune their intrinsic properties, such as reactivity, solubility, doping, and even the band structure, can improve their employability in different fields, such as electronic devices, biosensors, and composite materials [2,3,4]. Recently, both covalent and non-covalent-based chemical functionalizations of graphene-like surfaces have been explored intensively [5,6,7,8]. Philipson and co-workers demonstrated that the electronic property of a graphene surface is modulated using self-assembled networks of alkylamines, with the chain length as the variable. As a result, the doping level is controlled by the density of the strong n-type dopant amine groups on the surface. In our previous study, we explored the modification of graphitic surfaces by electroactive viologen molecules. According to the nature of n-doping, uncharged dibelzylviologen (DBV0)-based self-assembly permanently manifests as n-doped graphene [9,10]. De Feyter and co-workers have employed dodecadehydrotribenzoannuiene (DBA) and isophthalic acid (ISA) molecules to pattern graphitic surfaces with tailored 2D porous assemblies [11,12,13]. Even though the physisorbed self-assemblies of organic molecules only alter the carrier concentration of graphene without impacting its material properties [14,15,16], these functionalized layers are not stable under realistic operation conditions.
Exploiting the formation of C-C bonds, covalent modification using aryl diazonium to form robustly modified graphene-like surfaces has been intensively investigated [17,18,19,20,21,22]. Following this approach, some drawbacks of graphene, such as their zero bandgaps, and poor solubility in most solvents, can be addressed. However, the formation of multilayers by the continuous attack of radicals on already grafted moieties, causing a low degree of functionalization, remains a key challenge in diazonium electrografting [23,24].
Attempts to control the density and thickness of grafted films, which are key aspects for fine-tuning the processability and performance of graphene-like materials, have been addressed using different methods [24,25]. Menanteau et al. demonstrated that a condensed thin film was formed on a glassy carbon electrode by adding radical scavengers such as 2,2-diphenyl-1-picrylhydrazyl (DPPH) during the electrochemical grafting process [26]. This approach was further developed by González et al. to control the thickness of NBD-based films on highly oriented pyrolytic graphite (HOPG) [27]. However, this method is sensitive to the nature of the substituent on the aryl group [28]. Consequently, multilayer formation occurred with respect to aryl diazonium ions bearing an activating group in the para position. Therefore, the use of DPPH as a radical scavenger is effective for deactivating group-substituted aryl diazonium ions. Other chemical reducing agents, such as ascorbic acid [29], iodide [30], and hypophosphorus acid [31], have recently been demonstrated to be efficient for the grafting of aryl units on carbon surfaces. Our previous study demonstrated that the stereo effect with the substitution of functional groups at the 3,5 positions on aryl compounds serves as an effective strategy to obtain monolayered diazonium films [17,32]. Unlike previous reports that used chemical reducing agents, the stereo effect of substituted functional groups played a role in suppressing the further attachment of radicals to already grafted moieties, leaving a covalently grafted monolayer behind.
In addition, the stability of organic layers adsorbed on graphene-like surfaces against elevated temperatures and other environmental changes is another interesting concern. Several attempts using both theoretical and experimental approaches have been made to demonstrate the effect of bonding between organic moieties and surfaces on this characteristic [17,33,34]. Nevertheless, as far as we are aware, this issue has not been examined at the molecular level with respect to adlayers displaying hydrophobic behavior.
In this study, to further develop our successful strategy, we performed electrografting of 3,5-bis(trifluoromethyl)benzenediazonium, named 3,5-TFD, on an HOPG electrode. The selection of this molecule is based on the following advantages: (i) individual 3,5-TFD molecules contain six fluorine atoms, which greatly facilitate visualization at the molecular level of electrografted molecules by electron probe techniques; (ii) the functional groups attached to this molecule are hydrophobic, which enhances the hydrophobicity of the graphene-like surfaces upon electrografting. The efficiency of this approach, which was evaluated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning tunneling microscopy (STM), and Raman spectroscopy, revealed that the degree of covalent functionalization of 3,5-TFD compounds varied as a function of the concentration of this molecule in the grafting solution. More interestingly, the electrografted layer of 3,5-TFD is sensitive to thermal annealing and is mechanically removable locally by the STM tip. These results provide a firm basis for the reversible surface engineering of graphene and other 2D materials using this chemisorbed functionalization approach.

2. Materials and Methods

Analytical-grade hydrochloric acid, 3,5-bis(trifluoromethyl)aniline (3,5-TFA), and sodium nitrite were procured from Sigma Aldrich and used without further purification. High-purity water (Milli-Q purification system, <3 ppm, >18 MΩ cm) was used for electrolyte preparation, including supporting (50 mM HCl) and working electrolytes (50 mM HCl + 3,5-TFA). These electrolytes were deoxygenated with argon (Ar) gas for several hours before use. Immediately before injection into the electrochemical cell, the working electrolytes were added to an excess amount of 0.1 M NaNO2, followed by stirring for 2 min to generate the corresponding diazonium (3,5-TFD) in situ, as displayed in Figure 1. HOPG samples (grade ZYB) were obtained from Advanced Ceramics Inc., Cleveland, OH, USA. Before each experiment, the HOPG electrode was freshly cleaved using scotch tape.
All electrochemical measurements were performed using a potentiostat (VSP; Bio-Logic Science Instruments, Singapore). The electrochemical modification of HOPG samples was carried out at room temperature (25 °C) in a home-built single-compartment three-electrode cell with a working electrode area of 38.5 mm2, with Pt wire and Ag/AgCl (saturated KCl) serving as the counter and reference electrodes, respectively. During the measurements, all the electrolytes were placed under a solvent-saturated Ar atmosphere. After modification, the 3,5-TFD-modified HOPG samples were rinsed with hot toluene, followed by Milli-Q water to remove any physisorbed species on the grafted surface, and finally dried in a stream of nitrogen.
The blocking effect and electrochemical impedance spectroscopy (EIS) were performed on pristine and HOPG electrodes covered with 3,5-TFD molecules in a 0.2 M Na2SO4 solution containing the redox molecule, that is, [Fe(CN)6]3−/4−.EIS data were collected in the range of 105−10−1 Hz with a 10mV root-mean-square amplitude potential perturbation.
All STM experiments were performed using a Molecular Imaging STM system operating in constant-current mode. STM tips were prepared by mechanically cutting Pt/Ir wires (80%/20%, diameter 0.25 mm). STM data analyses were performed using WSxM 5.0 [35].

3. Results

3.1. Concentration-Dependent Electrografting of HOPG Electrode

First, the electrografting efficiency of 3,5-TFD on the HOPG electrode, a model multilayered graphene surface, was tested using the CV approach in an aqueous solution of 50 mM HCl + 1 mM 3,5-TFA + access 0.1 M NaNO2 (Figure 1a). As a result, a broad irreversible reduction peak situated at around E = −0.21 V vs. Ag/AgCl was observed and characterized. Significantly, the current observed in subsequent cycles decreased proportionally, which may indicate either a more gradual modification of the electrode surface or that the surface modification does not block further electron transfer [17]. This cathodic peak was assigned to the reduction of the 3,5-TFD cations and the formation of the corresponding aryl radicals that attack either the HOPG surface directly or the early-grafted molecules [17,36]. The detailed mechanism describing (i) the in-situ generation of 3,5-TFD cations, followed by (ii) the electroreduction and grafting process under electrochemical control, is summarized in Figure 1c [17,36].
The CV results demonstrated the complete electroreduction of 3,5-TFD molecules at a potential that is more negative than E = −0.21 V vs. Ag/AgCl. Thus, chronoamperometry (CA) was employed to investigate the relation between the functionalization degree of graphene-like surfaces and the concentration of 3,5-TFD molecules electrografted at a fixed potential of E = −0.3 V vs. Ag/AgCl (Figure 1b). After comparison, it was inferred that the current intensity of CA curves is increased with increasing consumption of 3,5-TFD concentrations. Accordingly, the charge transfer corresponding to individual concentrations was roughly estimated based on the intensity of the reduction peaks. As a result, a full monolayer was calculated with respect to the functionalized HOPG surface electrografted using a solution containing 2 mM 3,5-TFD, whereas sub-monolayers with decreased coverage were considered for the lower concentrations.

3.2. Electrochemical and Structural Properties of the 3,5-TFD-GraftedHOPG

The electrochemical and structural properties of 3,5-TFD-electrografted HOPG electrodes were verified using a compact toolbox of state-of-the-art techniques, including CV, EIS, STM, and Raman spectroscopy.
The blocking effect of concentration-dependent 3,5-TFD-functionalized HOPG samples was characterized by CV measurements using aelectroactive redox probe [Fe(CN)6]3−/4− (Figure 2a). With respect to the pristine HOPG electrode, an apparent quasi-reversible redox pair of peaks is observed at Ec = +0.16 V and Ea = +0.24 V vs. Ag/AgCl, respectively, as displayed by the dark blue curve. In contrast, the inhibition effect became increasingly effective, and the redox pair of the peaks was gradually suppressed with increasing concentrations. This observation is in good agreement with the charge transfer and surface coverage approximately calculated using electrochemical responses.
The electrochemical properties of pristine HOPG and concentration-dependent 3,5-TFD-modified HOPG electrodes were further investigated using EIS, measured at1 mM K3Fe(CN)6 + 0.2 M Na2SO4, as shown in Figure 2b. The charge transfer resistances of the individual electrodes correspond to the diameter of the semicircles in the Nyquist plot. The charge transfer resistance of pristine HOPG is small, owing to its conductive nature (dark blue curve and inset figure). A stepwise increase in the charge transfer resistance compared to that of the pristine HOPG was recorded with respect to the electrodes electrografted by 3,5-TFD, with its concentration as a variable. This indicates that the surface charge of the 3,5-TFD-grafted layers is governed by their surface coverage. Based on the obtained electrochemical data, a tentative equivalent circuit is proposed, as shown in Figure 2b, consisting of the solution resistance of the electrolyte (Rsol), diffusion impedance within the grafted 3,5-TFD layer (Rgl; Cgl), interfacial charge transfer resistance (Rct), double-layer capacitance (Cdl) at the grafted 3,5-TFD layer/HOPG interface, and Warburg impedance (Zw) [37,38].
The covalent bond between the grafted 3,5-TFD molecules and the graphene-like surfaces was determined by Raman spectroscopy, which directly provides information related to the surface defects formed on graphene-like surfaces. Theoretically, the D-peak appearing in the Raman spectra of graphene-like materials is correlated to lattice defects, including the formation of sp3 hybridization upon covalent grafting. The integrated intensity ratio of the D and G peaks (ID/IG) enables the verification of the concentration of covalent defect sites. Therefore, this is a standard method to characterize the degree of covalent functionalization via electrografting with the diazonium reagents [39,40]. Figure 3 shows the Raman spectra recorded on the HOPG surface before (black curve) and after grafting with 3,5-TFD molecules at different concentrations. Notably, all Raman spectra in this section were averaged over five sample spots. The Raman spectrum of pristine HOPG shows only two typical peaks at 1576 and 2679 cm−1,which are assigned to G and 2D bands of the carbon lattice, respectively [17]. The D band peak was featureless in this case, indicating that the HOPG surface did not contain any significant defects. More importantly, the D band peak was not prominent during scanning; hence, the laser-induced damage at the employed power levels can be eliminated. After being electrografted by 3,5-TFD molecules, coupled with the two peaks characterized for pristine HOPG, an additional feature located at 1336 cm−1 was observed. This peak was assigned to the D band activated exclusively upon introducing defects into the sp2 carbon lattice [41]. Careful analysis shows that the relative D band intensity increases with increasing concentrations and becomes saturated upon employing an approximate concentration of 2 mM. This implies an increase in the molecular density of 3,5-TFD covalently grafted onto the HOPG surface, until it reached a maximum value of 2 mM.
Together with Raman spectroscopy, the density of covalent anchoring sites formed on the HOPG surface can be directly visualized using regular STM, which is sensitive to the local density of states (LDOS) on the surface (Figure 4). Consequently, the surface coverage of grafted species was dependent on the 3,5-TFD concentration dissolved in the grafted solution. With respect to the 0.01 mM 3,5-TFD employed, the HOPG surface was partially covered by 3,5-TFD radicals, resulting in the formation of very low surface coverage, as shown in Figure 4a. By increasing the 3,5-TFD concentrations to 0.1 mM and 1 mM, higher amounts of grafted molecules on HOPG were observed (Figure 4b,c). This correlates with the higher D band intensity observed in the Raman spectrum. Upon employing a solution containing 2 mM 3,5-TFD, most of the HOPG surface was modified by these molecules, and hence, tip convolution prevented the acquisition of the cluster density (Figure 4d). Thus, by using different concentrations of 3,5-TFD molecules containing a working electrolyte for the electrografting process, it was demonstrated that the surface coverage of the grafted species varies as a function of the consumed 3,5-TFD concentration. A full monolayer of 3,5-TFD molecules formed on the HOPG surface at a concentration of 2 mM.

3.3. Global vs. Local Defunctionalization of the 3,5-TFD-Grafted Layer

The stability of the 3,5-TFD layer covalently grafted onto HOPG was characterized at both the local and global scales by employing STM tip-assisted nanoshaving and thermal annealing, respectively (Figure 5). Figure 5a shows an STM image of the 3,5-TFD-functionalized HOPG surface grafted with a solution of 2mM, showing a fully grafted layer of 3,5-TFD molecules. This grafted layer was found to be stable after several scans under mild tunneling imaging conditions, that is, a low tunneling current and high bias voltage.
For local defunctionalization, an STM tip was employed to locally remove the grafted 3,5-TFD species by applying drastic tunneling conditions, that is, a low bias voltage and high tunneling current. This resulted in the formation of a nanocorral free from grafted molecules, as marked by the dashed square, shown in Figure 5b. Our experimental results demonstrate that covalently grafted 3,5-TFD molecules can be locally defunctionalized from HOPG, leaving the pristine HOPG surface behind.
Defunctionalization at the global scale of the 3,5-TFD-grafted HOPG was also effectively demonstrated upon thermal annealing. Accordingly, the 3,5-TFD-grafted HOPG surface was thermally treated at 170 °C for 10 min under ambient conditions, followed by STM imaging. Consequently, a molecule-free HOPG surface was observed (Figure 5c), implying that the 3,5-TFD molecules were thermally detached from the HOPG surface upon heating. This implies that the carbon lattice reverts from sp3 to sp2 hybridization because of the breaking of the C-C covalent bonds formed by the electrografting of 3,5-TFD radicals. The reversion kinetics occurring on the functionalized HOPG surface is a function of the applied temperature, as verified by Raman spectroscopy (Figure 5c). Based on the ID/IG ratio, the loss of covalently bound molecules upon heating can be quantitatively evaluated. Accordingly, ID/IG of the grafted sample falls to 18.5% of its starting value upon heating to 150 °C and is almost eliminated entirely at 170 °C (Insert Figure 5c). This means that the D band intensity is gradually suppressed by the increased heating temperature, and becomes featureless when the heating temperature reaches 170 °C and beyond, which reflects defect-free sp2 hybridization. The Raman results were consistent with the STM observations shown in Figure 5c. Our findings propose that these are realizable pathways to defunctionalize the HOPG surface at both the local and global scales.

4. Conclusions

We have demonstrated that HOPG can be covalently electrochemically functionalized using 3,5-TFD. The efficiency of this approach was systematically examined using a state-of-the-art toolbox, including CV, EIS, STM, and Raman spectroscopy. The degree of functionalization could be tuned by varying the concentration of 3,5-TFD dissolved in the grafting electrolyte and reached the maxima at CM= 2.0 mM. The stability of the covalently functionalized layer of 3,5-TFD against elevated temperatures and mechanical force was demonstrated experimentally. This layer was either locally degrafted by the STM tip or globally detached by the thermal treatment, leaving defect-free graphitic surfaces behind. Both techniques facilitated the restoration of the carbon lattice from sp3 back to sp2 hybridization at the local and global scales. The obtained results offer a novel approach to reversibly and robustly functionalized graphene and other 2D materials for multiple uses in high-end applications.

Author Contributions

Conceptualization, T.M.T.H. and T.H.P.; methodology, T.M.T.H. and D.D.N.; validation, D.D.N. and N.H.H.; investigation, T.M.T.H. and N.H.H.; data curation, D.D.N. and N.H.H.; writing—original draft preparation, T.M.T.H.; writing—review and editing, T.H.P.; visualization, T.M.T.H. and D.D.N.; supervision, T.H.P.; project administration, T.H.P.; funding acquisition, T.H.P. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by the Ministry of Education and Training of Vietnam under grant number B2020-DQN-04.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing not applicable.

Acknowledgments

The authors thank Steven De Feyter and co-workers (KU Leuven, Belgium), for access to the scanning tunneling microscope and Raman microscope.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a,b) Cyclic voltammogram results illustrating the electrografting of 1mM 3,5-TFD on HOPG exhibit a well-defined cathodic peak at E = −0.21 V vs. Ag/AgCl assigned to the reduction of 3,5-TFD molecule to its radicals; (b) concentration-dependent electrochemical behavior of 3,5-TFDvia CA approach electrografted at E = −0.3 V vs. Ag/AgCl for 60 s; (c) mechanisms of (i) the in-situ formation of 3,5-TFD cation from its corresponding aniline, and (ii) electrografting of this molecule on graphene-like surface.
Figure 1. (a,b) Cyclic voltammogram results illustrating the electrografting of 1mM 3,5-TFD on HOPG exhibit a well-defined cathodic peak at E = −0.21 V vs. Ag/AgCl assigned to the reduction of 3,5-TFD molecule to its radicals; (b) concentration-dependent electrochemical behavior of 3,5-TFDvia CA approach electrografted at E = −0.3 V vs. Ag/AgCl for 60 s; (c) mechanisms of (i) the in-situ formation of 3,5-TFD cation from its corresponding aniline, and (ii) electrografting of this molecule on graphene-like surface.
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Figure 2. (a) Cyclic voltammograms and (b) Nyquist plots recorded with respect to pristine HOPG and HOPG electrografted with 3,5-TFD at different concentrations and brought into contact with 1 mM K3Fe(CN)6 + 0.2 M Na2SO4 electrolyte; inset plot in the range from 0 to 600 Ω showing the detailed EIS result of pristine HOPG; scan rate: 50 mV s−1.
Figure 2. (a) Cyclic voltammograms and (b) Nyquist plots recorded with respect to pristine HOPG and HOPG electrografted with 3,5-TFD at different concentrations and brought into contact with 1 mM K3Fe(CN)6 + 0.2 M Na2SO4 electrolyte; inset plot in the range from 0 to 600 Ω showing the detailed EIS result of pristine HOPG; scan rate: 50 mV s−1.
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Figure 3. Raman spectra of pristine HOPG and 3,5-TFD-grafted HOPG showing an evolution of the D band peak with respect to the increased concentrations. The spectra were averaged over 5 sample spots. Integration time: 100 s.
Figure 3. Raman spectra of pristine HOPG and 3,5-TFD-grafted HOPG showing an evolution of the D band peak with respect to the increased concentrations. The spectra were averaged over 5 sample spots. Integration time: 100 s.
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Figure 4. High-resolution STM images recorded with respect to samples grafted by different 3,5-TFD concentrations. Tunneling parameters (a) Iset = 0.1 nA, Vbias = 0.5 V; (b) Iset = 0.12 nA, Vbias= 0.6 V; (c) Iset = 0.08 nA, Vbias = 0.5 V; (d) Iset = 0.12 nA, Vbias = 0.7 V.
Figure 4. High-resolution STM images recorded with respect to samples grafted by different 3,5-TFD concentrations. Tunneling parameters (a) Iset = 0.1 nA, Vbias = 0.5 V; (b) Iset = 0.12 nA, Vbias= 0.6 V; (c) Iset = 0.08 nA, Vbias = 0.5 V; (d) Iset = 0.12 nA, Vbias = 0.7 V.
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Figure 5. STM images recorded with respect to the 3,5-TFD-grafted HOPG surfaces (a) before and after degrafting: (b) locally by STM tip scratching and (c) globally by thermal treatment. Tunneling parameters: (a,b) Iset = 0.08 nA, Vbias = 0.5 V; (c) Iset = 0.1 nA, Vbias = 0.6 V. (d) Raman spectra of 3,5-TFD-grafted HOPG thermally treated at different temperatures for 10 min under ambient conditions. The spectra were averaged over 5 sample spots. Integration time: 100 s; insert shows the temperature-dependent ID/IG ratio.
Figure 5. STM images recorded with respect to the 3,5-TFD-grafted HOPG surfaces (a) before and after degrafting: (b) locally by STM tip scratching and (c) globally by thermal treatment. Tunneling parameters: (a,b) Iset = 0.08 nA, Vbias = 0.5 V; (c) Iset = 0.1 nA, Vbias = 0.6 V. (d) Raman spectra of 3,5-TFD-grafted HOPG thermally treated at different temperatures for 10 min under ambient conditions. The spectra were averaged over 5 sample spots. Integration time: 100 s; insert shows the temperature-dependent ID/IG ratio.
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MDPI and ACS Style

Huynh, T.M.T.; Nguyen, D.D.; Hoang, N.H.; Phan, T.H. Reversible Tuning of Surface Properties of Graphene-like Material via Covalently Functionalized Hydrophobic Layer. Crystals 2023, 13, 635. https://doi.org/10.3390/cryst13040635

AMA Style

Huynh TMT, Nguyen DD, Hoang NH, Phan TH. Reversible Tuning of Surface Properties of Graphene-like Material via Covalently Functionalized Hydrophobic Layer. Crystals. 2023; 13(4):635. https://doi.org/10.3390/cryst13040635

Chicago/Turabian Style

Huynh, Thi Mien Trung, Duy Dien Nguyen, Nhat Hieu Hoang, and Thanh Hai Phan. 2023. "Reversible Tuning of Surface Properties of Graphene-like Material via Covalently Functionalized Hydrophobic Layer" Crystals 13, no. 4: 635. https://doi.org/10.3390/cryst13040635

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