1. Introduction
Recently, two-dimensional (2D) materials have received increased attentions because of their potential applications in next-generation electronic and optoelectronic devices [
1,
2,
3,
4,
5,
6]. As a typical
p-type material, GaTe has been widely used in random lasers and photodetectors, and for water splitting [
7,
8,
9,
10,
11]. Meanwhile, to achieve the applications, all kinds of GaTe nanostructures, including 1D nanowires, 2D nanoflakes, and thin film, have been fabricated by physical vapor deposition (PVD), chemical vapor deposition (CVD), or molecular beam epitaxy (MBE) [
12,
13,
14,
15,
16]. However, most of the fabricated GaTe samples crystallize in a monoclinic system with low symmetry
space group. Compared to the
m-GaTe,
h-GaTe is a metastable phase, which generally exists in ultrathin samples. To obtain
h-GaTe, over the previous decades, a lattice-induced strategy has been tried on Si (111), GaAs (111),
c-sapphire, or mica substrates. Among them,
h-GaTe can be grown on
c-sapphire or mica substrates in a narrow temperature growth region and shows a low yield [
17,
18,
19]. What is more, the reproducibility for the growth of
h-GaTe in these experiments is still a big challenge. Hence, a reliable and highly efficient synthesis method for the
h-GaTe is still expected.
In this work, the h-GaTe 2D nanosheets were successfully fabricated by a hexagonal ZnO-induced crystal growth strategy. Utilizing hexagonal ZnO nanocrystals as the substrate, the high-quality h-GaTe was grown by the simple CVD method under ambient pressure. It is worth noting that the synthesis has a high yield, and the hexagonal ZnO-induced h-GaTe growth shows a high reproducibility. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images indicate that the formed flower-like h-GaTe nanosheets are assembled by 2D GaTe triangles. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-resolution TEM (HRTEM) uncover the hexagonal crystal structure in the obtained GaTe nanosheets. Meanwhile, the electron localization function simulation confirms the lattice-induced crystal growth mechanism for the growth of h-GaTe on hexagonal ZnO. The h-GaTe nanosheets show a p-type characteristic. In addition, the electronic structure of h-GaTe and the interfacial properties of GaTe/ZnO heterostructure were also investigated by first-principle calculations.
3. Results and Discussion
Figure 1a outlines the two-step procedure adopted to grow the flower-like GaTe nanosheets. The ZnO nanocrystals were grown on the Si wafer via the first growth step (
Figure 1b), and GaTe flower-like nanosheets were synthesized on the ZnO/Si wafer via the second growth step (
Figure 1c). Details of the growth process can be found in the Experimental Section. The ZnO nanocrystals with hexagonal symmetry as the buffer layer promoted the growth of
h-GaTe (with triangular morphology). Furthermore, the high-magnification SEM images (
Figure 1d,e) clearly reveal that the grown GaTe nanosheets are well aligned and integrated with ZnO nanocrystals. The GaTe nanosheets are distributed horizontally on the ZnO surface, forming a flower-like morphology. The GaTe nanosheets prefer to grow around the (001) basal plane of ZnO due to its lower surface energy [
23]. Further energy dispersive spectroscopy (SEM-EDS) for the ZnO nanocrystals and GaTe nanosheets reveals Zn, O, Ga, and Te signals, confirming their elemental composition (
Figure S1, see Supplementary Materials).
Figure 1f shows an atomic force microscopy (AFM) image of the GaTe nanosheets, which exhibits a clearly-layered structure with a multi-layered feature, implying that the flower consists of GaTe nanosheets with different layer numbers. As shown in
Figure 1g, the thickness of the GaTe triangle is ~4.7 nm, which corresponds to six layers because one single-layer GaTe has a thickness of ~0.8 nm [
16].
XPS analysis was performed to investigate the chemical composition, as shown in
Figure 2a, presenting the XPS spectrum of the GaTe nanosheets. Ga and Te elements are from GaTe grown in the second growth step and Zn and O from the ZnO nanocrystals serving as nucleation sites in the first step. In the XPS spectrum of Zn (
Figure 2b), there emerge strong peaks at 1045.4 and 1022 eV, which correspond to Zn 2p
1/2 and Zn 2p
3/2 in ZnO, respectively. The Zn peaks are attributed to the ZnO molecular environment [
24]. The Ga 3d peak in
Figure 2c is located at 20.2, while a shoulder at 22.6 eV is observed and has been explained by an emission of the oxygen 2s level [
9]. In addition, as shown in
Figure 2d, there are two distinct Te 3d3/2 and 3d5/2 peaks at 582.5 and 572.2 eV, which correspond to those of GaTe [
25]. The appearance of peaks at 586.7 and 576.6 eV indicates the presence of TeO
2, mainly due to the oxidation of Te [
26].
To investigate the crystallinity of the as-prepared samples, XRD analysis of the GaTe nanosheets is shown in
Figure 3a. The diffraction peaks at
2θ = 31.8°, 34.3°, 36.5°, and 57.2° correspond to the (100), (002), (101), and (102) planes of the ZnO hexagonal phase (JCPDS 00-001-1136,
a = 0.324 nm and
c = 0.517 nm), respectively [
27,
28]. In addition, there emerge diffraction peaks at 2
θ = 25.3°, 33.1°, and 56.6°, which can be attributed to the (100), (104), and (204) planes of GaTe, respectively. The XRD pattern is well consistent with that of
h-GaTe (JCPDS no. 04-003-6485 (P63/mmc)), confirming that the as-synthesized nanosheets have a
h-GaTe phase [
29]. To further conduct TEM observation to probe the crystallinity of the GaTe nanosheets, the ultrasonic-treated nanosheet sample was transferred onto a quantifoil copper TEM grid [
30,
31], as shown in
Figure 3b.
Figure 3c shows a high-resolution TEM (HRTEM) image of a circle area in
Figure 3b, in which a hexagonal arrangement can be clearly observed. A further selected area electron diffraction (SAED) pattern (
Figure 3d) of the 2D GaTe triangle exhibits a pattern with six-fold symmetry diffraction spots, confirming the single-crystalline nature of GaTe nanosheets. The spot pattern can be well indexed to
h-GaTe (JCPDS no. 01-089-2675 (P63/mmc)) with lattice parameters of
a = 0.406 nm and
c = 1.696 nm [
32,
33].
To examine the quality of the flower-like GaTe nanosheets, Raman spectrum and intensity map** was conducted to investigate the lattice vibration of the GaTe nanosheets (
Figure 4a) [
34]. As shown in
Figure 4b, six Raman peaks are identified approximately at 102, 126, 143, 330, 383, and 438 cm
−1. The peaks at 126 and 143 cm
−1 correspond to the
Ag modes of GaTe [
35,
36,
37]. The peak at ~102 cm
−1 reflects the
E2L line from the wurtzite ZnO nanocrystals, which is derived from the zone folding of the transverse acoustic (
TA) mode in ZnO [
38]. The peak at 383 cm
−1 corresponds to
A1T and that at 438 cm
−1 to
E2H of ZnO. The peak at ~330 cm
−1 is attributed to the second-order Raman processes, which corresponds to
E2H-2L of ZnO [
39]. To investigate the spatial variation of the peak intensity, Raman intensity map** of the flower-like GaTe nanosheets (see
Figure 4a) was performed, as shown in
Figure 4c, where one can notice the Raman intensity map of the
Ag mode (~126 cm
−1) of GaTe with an intensity distribution over the surface of the GaTe nanosheets. Obviously, the high intensity region (the bright spot at the center) corresponds to the area covered with GaTe nanosheets.
DFT is typically used to predict structures and calculate band information [
40,
41].
Figure 5a shows the side view of ABC-stacked
h-GaTe, and the calculated band gap structure is shown in
Figure 5b. The conduction-band minimum (CBM) is located at 0.37 eV, and the valence-band maximum (VBM) is located at 0.26 eV. Obviously, the results demonstrate that
h-GaTe is a
p-type semiconductor with an indirect bandgap. To further investigate the electron localization at the interface of the ZnO/GaTe heterostructure, an electron localization function simulation is employed. The heterostructure and calculated results are shown in
Figure 5c, showing a stronger interaction in the interface of the ZnO/GaTe heterostructure compared to van der Waals force in the interlamination of pure GaTe. According to the DFT results, we attribute the structural transformation to the strongly interfacial interaction, together with the hexagonally symmetric ZnO seed layer.
One of the most important advantages of GaTe over other 2D semiconductors is the
p-type conductivity. Furthermore, ZnO is known as an intrinsic
n-type semiconductor. Hence,
p-type GaTe can naturally form heterostructure
p-n junctions with
n-type ZnO, which may enable many optoelectronics applications. Here we employed a conductive atomic force microscope (CAFM) technique to measure the vertical charge transport of GaTe/ZnO. The geometry of the experimental setup is shown in the inset of
Figure 5d. A conductive Pt/Ir-coated Si tip served as positionable tip to the surface of the GaTe nanosheet flower, which forms an Ohmic contact to the GaTe nanosheet flower due to the package materials (Pt/Ir) with high work function and the tight contact force between the probe and sample under the CAFM mode [
39]. Silver (Ag) paste is used as the second contact to the
n-type ZnO film to obtain Ohmic contact, enabling the current through the
p-GaTe/n-ZnO heterojunction diodes [
42]. The CAFM measurements demonstrate that the as-grown sample has a slightly stronger rectifying feature, with typical I-V characteristics shown in
Figure 5d.