Synthesis and Characterizations of 2D Platinum Diselenide ^†

Abstract

Platinum diselenide (PtSe₂), which belongs to the transition metals dichalcogenide (TMDCs) class of 2D materials, is characterized with a transition from semimetal to semiconductor with a thickness variation from bulk to monolayer and found in versatile applications especially in sensors and mid-infrared detectors. In this study we report the large-scale synthesis of PtSe₂ layers by thermally assisted selenization of pre-deposited platinum films in a horizontal quartz-tube Chemical Vapor Deposition (CVD) reactor. Raman spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used for characterization of the obtained 2D PtSe₂. It is observed that the Raman spectra of PtSe₂ show strong dependence on the thickness (Pt deposition time). XPS analysis was applied to examine the chemical compositions in order to assess the quality of the synthesized PtSe₂ films. All the studied properties reveal great potential to obtain continuous layers with a controlled thickness and composition and further potential for integration in functional heterostructures for future nanoelectronic and optoelectronic devices.

1. Introduction

Noble TMD (nTMD) 2D materials are displaying many fascinating properties including a widely tunable bandgap, moderate carrier mobility, anisotropy and high air stability [1,2,3].

The nTMDs, and particularly the platinum diselenide PtSe₂, show diverse chemical and physical properties which are intensively studied especially in a 2D form [4]. Their complex electronic structure with a notable indirect band gap can be tuned by several factors, such as varying the number of layers [5], strain [6] or induced defects [7]. In monolayer form, PtSe₂ is semiconducting with a band gap in the range 1.2–1.8 eV [2]. It turns into a semimetal by increasing the number of layers. Moreover, the charge–carrier mobility of PtSe₂ (values of 3000 cm²/V·s [4]) is among the highest in TMDCs and is comparable to black phosphorus (1000 cm²/V·s [8]) which makes it a promising candidate for high performance gas-sensing and photovoltaics, high-speed electronics and optoelectronics. PtSe₂ films grown by the thermally assisted conversion of platinum at a back-end-of-the-line complementary metal–oxide-semiconductor (CMOS) compatible temperatures show excellent promise for future applications [9].

Another notable advantage of PtSe₂ is the environmental stability and appropriate synthesis requirements which are compatible with the conventional industrial technologies. The established synthesis approaches such as Thermally Assisted Conversion (TAC) [4] and other methods such as Chemical Vapor Deposition [10] and Molecular Beam Epitaxy [11] show fine options for PtSe₂ nanostructure controllability and engineering for applications in opto-electronics, catalysis and sensors [12,13,14].

The present study systematizes the initial technological steps of TAC synthesis of PtSe₂ and the corresponding quality verification via structural and chemical characterization by XRD, Raman spectroscopy and XPS.

2. Materials Preparation and Methods

PtSe₂ films were synthesized by a twostep TAC procedure [4]. The first (i) preparation stage facilitated a pre-deposition of Pt film using in a custom-built magnetron sputtering system with 3″ diameter Pt target (99.99% purity) by an RF (radio-frequency) magnetron sputtering process. The sputtering gas was argon of 99.999% purity. The applied power was set to 300 W and the deposition pressure was 6 × 10⁻¹ torr. The substrate holder was rotating during the sputtering run. The thickness of the deposited Pt films was controlled by the sputtering time. Pt thin films of varying thicknesses were deposited onto SiO₂/Si and the fused quartz substrate.

The second (ii) synthesis stage—a direct selenization of the pre-deposited films in a Chemical Vapor Deposition (CVD) reactor based on a dual zone tube furnace. The Se vapor (precursor powder) source and the Pt films on Si/SiO₂ are inserted as follows: in a low temperature zone (~220 °C) and in the growth high temperature zone (~550 °C), respectively. The process is mediated via a carrier/reactive gas mixture flow of 95% Ar/5% H₂ for 2 h with consequent formation of another necessary gaseous precursor—H₂Se to enable the PtSe₂ growth. A schematic of the CVD reactor is shown in Figure 1.

The focus of the study is the synthesis details, structural and chemical characterizations of PtSe₂ layers obtained from a 30 s Pt predisposition time. The X-ray diffraction patterns were collected using a Bruker D8 Advance diffractometer with Cu Kα radiation and a LynxEye detector within the range of 5.3° to 80° 2θ, with a constant step of 0.02°. The XPS analysis was performed using a Kratos AXIS Supra spectrometer with a non-monochromatic Al X-ray source under vacuum (<10⁻⁸ Pa) at a 90 take-off angle. The acquired photoelectron spectra were additionally processed using background signal subtraction and fitting procedures routines of the XPSPEAK41 software package. Raman spectroscopy measurements were carried out on an 80 cm Dilor XY-800, with a triple monochromator allowing for multi-channel liquid nitrogen cooled CCD (charge-coupled device) detection or single-channel PM detection. The experiments were performed at room temperature using 514.5 nm excitation line of a CW Ar⁺ laser.

3. Results and Discussion

The physical nature of 2D materials, such as TMDs and particularly PtSe₂, is determined by the layered crystal structure (Figure 2) with van der Waals stacking and strong anisotropy of the properties. The XRD pattern identify the PtSe₂ film crystal lattice with unit cell parameters determined to be a = 3.728 Å and c = 5.06 Å with hexagonal P3m1 (164) space group. The main (001) characteristic peak is detected approximately at 17.6° degrees 2θ. In the inset section an enlarged diffractogram sector (in the range 20–80°) is presented, showing the other peaks with diminished intensity as a consequence of the layered structure and dominant (001) orientation—implying a high crystalline quality and c-axis growth of the film plane. The peaks are assigned to (002), (003) and (004) crystallographic orientation and negligible traces (<1%) of Pt and the substrate SiO₂/Si (111) are also identified.

The chemical composition and binding energies of the PtSe₂ film were verified also by XPS. The spectrum (Figure 3) shows the presence of both Pt and Se and the analysis data indicate that the PtSe₂ phase is successfully formed. The Se 3d peak is deconvoluted into two contributions: the main from PtSe₂ appears at around 55 eV (the spin-orbital splitting between Se 3d_5/2 and Se 3d_3/2 is 0.86 eV) and the minor with a binding energy around 59.5 eV typical for Se oxide. There are also traces for metallic state Pt in this range.

The Pt 4f is deconvoluted into two contributions; the one at ~72.3 eV is attributed to PtO_x/hydroxide while the second one at ~73.6 eV was attributed to PtSe₂. The spin-orbital splitting at 3.35 eV was used for the fitting procedure.

Raman spectroscopy is a powerful and nondestructive characterization technique which is widely used for 2D materials. The Raman spectra of TMDs are generally characterized by two main peaks corresponding to the in-plane and out-of-plane motions of atoms. The two Raman-active modes in the spectra of PtSe₂ are labeled as A_1g and E_g [16]

A schematic of the vibrational modes in PtSe₂ is shown in Figure 4, with arrows drawn as guides to show the origin of each mode on the phonon dispersion curve. These include the E_g mode, describing the in-plane vibration of selenium atoms in opposite directions within a single layer, and the A_1g mode describing the out-of-plane vibration of selenium atoms. The characteristic Raman active Eg (178 cm⁻¹) and A1g (208 cm⁻¹) mode of TAC deposited PtSe₂ confirm the composition and quality of the obtained samples.

4. Conclusions

The layered 2D material PtSe₂ was successfully synthesized by a thermal assisted conversion process. The structural and chemical characterizations of the obtained PtSe₂ confirm the composition and crystalline quality. The obtained results allow further directions for improvement of the preparation procedure (in particular the deposition periods) to facilitate the nanostructure synthesis approach towards the 2D PtSe₂ applications.