Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Precise Layer-Dependent Electronic Structure of MBE-Grown PtSe$_2$   Two-dimensional (2D) platinum diselenide (PtSe$_2$) has received significant
attention for 2D transistor applications due to its high mobility. Here, using
molecular beam epitaxy, we investigate the growth of 2D PtSe$_2$ on highly
oriented pyrolytic graphite (HOPG) and unveil their electronic properties via
X-ray photoelectron spectroscopy, Raman spectra, and scanning tunnelling
microscopy/spectroscopy as well as density functional theory (DFT)
calculations. PtSe$_2$ adopts a layer-by-layer growth mode on HOPG and shows a
decreasing band gap with increasing layer number. For the layer numbers from
one to four, PtSe$_2$ has band gaps of $2.0 \pm 0.1$, $1.1 \pm 0.1$, $0.6 \pm
0.1$ and $0.20 \pm 0.1$ eV, respectively, and becomes semimetal from the fifth
layer. DFT calculations reproduce the layer-dependent evolution of both the
band gap and band edges, suggest an indirect band-gap structure, and elucidate
the underlying physics at the atomic level.

19
The categories of this paper are: eess
25
The categories of this paper are: patt-sol
2
The categories of this paper are: math
8
The categories of this paper are: hep-ex
27
The categories of this paper are: comp-gas
20
The categories of this paper are: acc-phys
22
The categories of this paper are: chao-dyn
14
The categories of this paper are: math-ph
24
The categories of this paper are: dg-ga
33
The categories of this paper are: plasm-ph
34
The categories of this paper are: chem-ph
37
The categories of this paper are: supr-con
13
The categories of this paper are: quant-ph
32
The categories of this paper are: atom-ph
29
The categories of this paper are: funct-an
15
The categories of this paper are: nucl-ex