Precise Layer‐Dependent Electronic Structure of MBE‐Grown PtSe₂

Abstract

layer-modulated magnetism and robust stability in air. These properties make it a promising candidate in various fields like electronics, [1][2][3][4][5] optoelectronics, [6][7][8][9][10][11][12][13] spintronics, [14] catalysis, [15] micro-electromechanics, [16] and sensing. [6,17,18] Monolayer or few-layer PtSe 2 can be synthesized by different methods, such as direct selenization of Pt films at a low temperature (≤400 °C), [3,4,6,8,19] which makes it scalable and compatible with current silicon chip fabrication technology, molecular beam epitaxy (MBE), [16,20,21] chemical vapor deposition (CVD), [5,22] and chemical vapor transport (CVT). [1,23] While PtSe 2 is a semimetal in bulk, [23] it becomes a semiconductor when thinned down to a few layers, due to the quantum confinement effect. [3] Its electronic structure has been studied by angle-resolved photoemission spectroscopy (ARPES), which reveals the semiconducting property of monolayer PtSe 2 with the top of its valence band located at 1.2 eV below the Fermi level. [17,20] Complemented by density functional theory (DFT) calculations under the localizeddensity approximation (DFT-LDA), monolayer PtSe 2 is determined to be an indirect-gap semiconductor (≈1.2 eV). [17] Besides the first layer, PtSe 2 remains semiconducting at the thickness of bilayer with a significantly reduced gap of 0.21 eV and becomes semimetallic from the third layer, as predicted by DFT-LDA calculations. [17,24] However, the ARPES measurement only gives the area-average information of the band structure below the Fermi level and its results are subject to the crystal size and quality. For

MBE-grown ultrathin PtTe2 films and their layer-dependent electronic structures

“…Due to the limitation of the STM scanning area, the crystal size can be found to be 700 × 700 nm −2 . The yield of bilayer PtTe 2 coverage is much higher than that reported before, 14 and more importantly our finding here is a rather different feature compared to a stack of different layers in previous work of other NTMDs, such as PtSe 2 , 14,20 PdSe 2 , 22 and PdTe 2 . 39 At 500 °C and 30 min (Fig.…”

“…The E g mode shows a major red shift with increasing growth temperature (or equivalently increasing thickness of crystal islands), whereas the A 1g mode remains unshifted, which may be attributed to the stacking-induced structural changes and long-range coulombic interactions, as observed in the thickness-dependent Raman spectra of PtSe 2 , MoS 2 , and PtTe 2 . 4,20,37,38 The major shift of E g and enhancement in the intensities of E g and A 1g from 350 to 400 °C are caused by the significant increase in the thickness of crystal islands, as shown in Fig. 2d.…”

“…1–3,17,21 This problem can be avoided or alleviated partially by the molecular beam epitaxy (MBE) method, in which metal atoms and chalcogen atoms are co-deposited onto a substrate maintained at a specific temperature with the growth rate precisely controlled. However, in most cases the obtained single crystal domains usually have a dimension smaller than 200 nm and mostly are a stack of different layers, as observed in MBE-grown PtSe 2 , 14,20 PtTe 2 , 14 PdSe 2 , 22,23 and PdTe 2 . 24 The precise layer-dependent electronic structure of PtTe 2 is still controversial.…”

Optical Characterization of Few-Layer PtSe2 Nanosheet Films

“…Experimentally, the band gap of PtSe 2 layers grown by molecular-beam epitaxy was recently measured by scanning tunneling spectroscopy (STS). 34 The values obtained from STS experiments were 0.6 eV and 0.2 eV for three and four layers, respectively, and 0 eV for layer numbers greater than five.…”

“…The monolayer is an indirect semiconductor with a calculated band gap of 1.2–1.5 eV. − The band gap rapidly decreases with increasing numbers of layers, and the samples as thick as 5–6 layers (3–4 nm) are already semimetallic. Experimentally, the band gap of PtSe 2 layers grown by molecular-beam epitaxy was recently measured by scanning tunneling spectroscopy (STS) . The values obtained from STS experiments were 0.6 eV and 0.2 eV for three and four layers, respectively, and 0 eV for layer numbers greater than five.…”