Direct synthesis of WSe2/PtSe2 heterostructures

The results of a successful synthesis of a WSe2/PtSe2 heterostructures are presented. High quality crystalline films were achieved through a one–step selenization of a pre-deposited tungsten film with pre-deposited platinum as an underlayer. The role of the PtSe2 layer, formed during selenization, was to assists the growth of crystalline WSe2. The existence of WSe2 was confirmed using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The crystallinity of the samples was investigated using X-ray diffraction (XRD). Surface measurements were performed using atomic force microscopy (AFM).


Introduction
The discovery of graphene showed the interesting and outstanding properties that 2D matter can possess [1,2].Since then, many families of 2D materials have been discovered among which is the prominent family of transition metal dichalcogenides (TMDCs).Research into TMDCs has revealed outstanding physical properties and broad prospects for applications [2][3][4].Transition metal dichalcogenides such as MoS2, PtSe2, WSe2 etc., are atomically thin layered van der Waals materials with a general formula of MX2, where M is a transition metal atom and X is a chalcogen atom, have shown potential for nanoelectronics, spintronics and optoelectronic device application [5][6][7].By their nature 2D materials have weak interaction with the growth substrate which lowers the requirements for lattice matching.This makes it easier to grow 2D materials epitaxially and produce a wide range of heterostructures for use in new technologies [8].
Platinum diselenide (PtSe2) is a prominent member of the noble transition-metal dichalcogenides (nTMDC) family.In bulk form PtSe2 behaves like a semimetal while in mono and few layer form it is a p-type semiconductor, [9] with high charge mobility and tuneable band gap [10].Tungsten diselenide (WSe2) is a p-type semiconducting 2D material.Bulk WSe2 is an indirect-bandgap (~ 1.2 eV) semiconductor while in monolayer form it transforms into one with a direct-bandgap of ~1.6 eV [11].WSe2 possesses strong spin-orbit coupling in comparison to other TMDCs [12,13] making it a viable candidate for spintronic applications.
The heterostructure was fabricated through direct selenization of a pre-deposited tungsten film over a pre-deposited platinum under layer.The role of the PtSe2 layer, formed during selenization, is to assists the growth of crystalline WSe2.

Growth for WSe2/PtSe2 heterostructure
Synthesis of the WSe2/PtSe2 heterostructure was done in one step CVD selenization of pre-deposited platinum and tungsten layers.Firstly, a thin layer of Pt (10 nm) was deposited on a pure silicon substrate via magnetron sputtering, afterwards the W (1 nm) layer was deposited via vacuum evaporation.Thickness of the platinum layer was determined using Atomic Force Microscopy (AFM) measurments and the thickness of the W was estimated using a quartz resonator during the deposition process.
The layer structure was put in the center of a quartz tube reactor figure 1.Before starting the synthesis process excess oxygen in the system was removed by passing 300 sccm Ar/H2 (9:1) mixture for 20 min.The temperature of the central zone was set to 680 o C and a selenium source was put upstream in a separate heating zone, approximately 15 cm from the central zone, and set to 280 o C. 150 sccm.Ar/H2 (9:1) gas mixture was used as a carrier gas.The duration of the process was 2h and the pressure one atmosphere.

Structural and optical characterization
Raman spectroscopy was performed using HORIBA Jobin Yvon Labram HR visible spectrometer with 633 nm excitation wavelength.Signal was collected using a 100x objective in a backscattering configuration with an 1800 lines/mm grating.The height profile and surface morphology measurements were done using AFM model MFD-3D Asylum Research.X-ray Photoelectron Spectroscopy (XPS) spectra were collected using a Kratos AXIS Supra spectrometer.X-ray diffraction (XRD) patterns were measured using a Bruker D8 Advance diffractometer.

Discussion
Raman spectroscopy was performed on the synthesized PtSe2/WSe2 heterostructure and the results are shown in figure 2. The spectrum contains the WSe2 fundamentals:  1 2 (in-plane vibration), which is centеred at ~248 cm -1 , and 1 (out-of-plane vibration) represented as a sharp peak at 251 cm -1 .For 2H WSe2 the  1 2 and 1 frequencies almost overlap [14].The intense peak 2A() located at 255 -260 cm -1 is attributed to second-order phonons and also shows strong layer dependence.Multiple second-order peaks belonging to WSe2 are located at ~137 cm -1 , ~242 cm -1 and ~375 cm -1 .The two peaks at 137cm -1 and 375 cm -1 belong to the combination modes A1g -LA(M) and E 1 2g + LA(M), respectively [15].The peak around ~300 cm -1 is attributed to the growth substrate (Si).Also present in the spectra are the in-plane (E2g) and out-of-plane(A1g) vibrational modes located at ~175 cm -1 and ~205 cm -1 , respectively, [16]   X-ray photoemission spectroscopy (XPS), shown in figure 3, was done in order to assess the chemical composition and binding energies of W4f and Se3d.The W4f spectra shows two prominent peaks at ~34.2 eV and ~32 eV which correspond to W4f5/2 and W4f7/2 respectively.The Se3d peak is ~ 54,6 eV.The binding energies of W4f and Se3d show they are in a bound state and confirm the existence of 2D WSe2 [17,18].Figure 3. XPS spectra of the WSe2/PtSe2 heterostructure for W4f (a) and Se3d (b) core levels.X-ray Diffraction (XRD) analysis reveals WSe2 with good crystallinity.The peak in figure 4 (a) is associated with WSe2 in (002) crystallographic orientation and was matched with that in reference file 00-038-1388 for WSe2.All of the grown WSe2 is in 2H phase and grows along the c-axis in a preferential orientation, with the van der Waals gaps parallel to the substrate [19].The peak at ~30 o 2θ is generated from the PtSe2 underlayer [17].Measurements performed using atomic force microscopy (AFM) reveal a generally smooth surface figure 4 (b) with an RMS of ~4.95 nm, which is beneficial for application in devices.The thickness of the heterostructure is measured to be around 15 nm figure 4 (c).

Conclusion
WSe2/PtSe2 heterostructure was directly synthesised from pre-deposited Pt and W layers. WSe2 was synthesized with the aid of a PtSe2 buffer layer in a simple one step selenization process at atmospheric pressure.The WSe2 was revealed to be in (002) crystallographic orientation and have good crystallinity, confirmed using XRD analysis.Confirmation of the existence of WSe2 and PtSe2 was done using Raman spectroscopy.XPS was also used to confirm the existence of 2D WSe2.Surface morphology and thickness measurements were performed using AFM revealing smooth surface with RMS of 4.95 nm.The smooth surface and the highly oriented WSe2 on the surface can be used in the fabrication of novel electronic and optoelectronic devices.
Infrastructures, supported by Bulgarian Ministry of Education and Science was in this investigation.
of the PtSe2 under layer.