Engineering 2D Materials from Single-Layer NbS2

Abstract

Starting from a single layer of NbS₂ grown on graphene by molecular beam epitaxy, the single unit cell thick 2D materials Nb_5/3S₃-2D and Nb₂S₃-2D are created using two different pathways. Either annealing under sulfur-deficient conditions at progressively higher temperatures or deposition of increasing amounts of Nb at elevated temperature result in phase-pure Nb_5/3S₃-2D followed by Nb₂S₃-2D. The materials are characterized by scanning tunneling microscopy, scanning tunneling spectroscopy, and X-ray photoemission spectroscopy. The experimental assessment combined with systematic density functional theory calculations reveals their structure. The 2D materials are covalently bound without any van der Waals gap. Their stacking sequence and structure are at variance with expectations based on corresponding bulk materials highlighting the importance of surface and interface effects in structure formation.

Keywords: covalent transformation; molecular beam‐epitaxy; niobium disulfide; single layer.

Figure 1

Concept of covalent transformation. a) Single‐layer H‐NbS₂ on Gr/Ir(111). b) Covalent transformation by heating and dissociation. c) Covalent transformation by deposition of additional Nb.

Figure 2

STM topographs of an isochronal annealing sequence of initial single‐layer NbS₂ islands without supply of additional S. Annealing time intervals are 360 s. a) Single‐layer NbS₂ islands after room temperature growth and annealing to 820 K. b–f) After additional annealing to (b) 920 K, (c) 1020 K, (d) 1120 K, (e) 1220 K, and (f) 1320 K. In (d) a bright spot at an island edge is encircled and a peninsula attached to an edge is highlighted by a white arrow. Height profiles along the black lines are shown below the topographs. Height levels d = 0.62 nm, d′ = 0.99 nm and d′ = 0.93 nm distinguish between single‐layer NbS₂, $\sqrt{3}\times \sqrt{3}$ ‐ phase, 1 × 1 ‐ phase, respectively. Image information: for all size is 150 nm × 90 nm, (a) V _s =1.0 V, I _t = 0.23 nA; (b) V _s = 0.95 V, I _t = 0.34 nA; (c) V _s = 1.0 V, I _t = 0.26 nA; (d) V _s = 0.92 V, I _t = 0.33 nA; (e) V _s = 1.0 V, I _t = 0.32 nA; (f) V _s = 2.2 V, I _t = 0.06 nA. In (f) a large tunneling resistance was chosen, to avoid tip sample interaction with the tall cluster.

Figure 3

Atomic resolution STM topographs of a) pristine single‐layer NbS₂, b) the $\sqrt{3}\times \sqrt{3}$ ‐ phase, and c) the 1 × 1 ‐ phase taken at 1.7 K. In the STM topographs the unit cells of the three phases are indicated by cyan rhomboids. Magenta rhomboid is the $(\sqrt{3}\times \sqrt{3})\mathrm{R}{30}^{\circ }$ superstructure. Height profiles along the black lines are shown below the topographs. Image information: for all size 6 nm × 6 nm and T _s = 1.7 K, (a) V _s = 50 mV, I _t = 0.5 nA; (b) V _s = 100 mV, I _t = 0.80 nA; (c) V _s = 100 mV, I _t = 0.70 nA.

Figure 4

a) XPS of the S 2p core level of initial single‐layer NbS₂ on Gr/Ir(111) transformed during annealing. After room temperature growth, for each spectrum the sample was annealed to the indicated temperature without supply of additional S and cooled down to 300 K for measurements. The spectra are grouped in three temperature ranges according to their similarities: yellow, green, and orange. b–d) S 2p core level spectra after annealing to (b) 820 K, (c) 1020 K, and (d) 1220 K fitted with components.

Figure 5

Formation of the $\sqrt{3}\times \sqrt{3}$ ‐ phase by Nb supply. a) Single‐layer NbS₂ grown by deposition of 0.36 ML Nb in S background pressure at room temperature and annealed to 820 K in the absence of additional S supply. The bright protrusion at the bottom of the image consists of an Ar‐filled Gr‐blister.^{[ 46 ]} b) Sample after deposition of additional 0.12 ML Nb at 820 K in the absence of additional S supply results in the formation of the $\sqrt{3}\times \sqrt{3}$ ‐ phase. A tiny piece of single‐layer NbS₂  is encircled. Inset: atomic resolution topograph of boxed area. Height profiles are taken along the black lines in the STM topographs. Image information: (a) size 100 nm × 100 nm, V _s = 1.0 V, I _t = 1.00 nA; (b) size 100 nm × 100 nm, V _s = 1.0 V, I _t = 1.0 nA; Inset: 5 nm × 5 nm, V _s = 0.1 V, I _t = 5 nA.

Figure 6

Formation of the 1 × 1 ‐ phase by Nb supply. a) Single‐layer NbS₂ grown by deposition of 0.33 ML Nb in S background pressure at room temperature and annealed to 820 K in the absence of additional S supply. b) Sample after deposition of additional 0.33 ML Nb at 820 K. Inset: atomic resolution topograph of the boxed area. c) Sample after additional annealing to 1020 K. Inset: atomic resolution STM topograph of the boxed area. Height profiles are taken along the black lines in the topographs. Image information: (a) size 100 nm × 100 nm, V _s = 1.0 V, I _t = 0.23 nA; (b) size 100 nm × 100 nm, V _s = 1.0 V, I _t = 0.3 nA; Inset: 5 nm × 5 nm, V _s = 0.1 V, I _t = 5 nA; (c) size 100 nm × 100 nm, V _s = 1.2 V, I _t = 0.3 nA; Inset: 5 nm × 5 nm, V _s = 0.1 V, I _t = 5 nA.

Figure 7

a) High‐resolution XPS of the S 2p core level of NbS₂ on Gr/Ir(111). b) After deposition of additional Nb at 820 K. c) After annealing at 1020 K. Fit of each spectrum with five S 2p components.

Figure 8

Chemisorption of Nb₂S₂‐2D to Gr. Relative total energy of minimum energy configuration of Nb₂S₂‐2D (lowest energy structure) as a function of the distance to Gr. Zero point of the energy scale is at 0.36 nm in the physisorbed state. Inset: side view ball models of relaxed DFT geometries for Nb₂S₂‐2D in the 0.36 and 0.22 nm Nb‐C distances. Nb atoms: cadet blue balls and gray; S atoms: yellow; dark gray: C atoms.

Figure 9

DFT calculated ball model representation of the 1 × 1 ‐ phase with stoichiometry Nb₂S₃‐2D in a) top and b) side view. Nb atoms: cadet blue balls and gray; S atoms: yellow; dark gray: C atoms. c) Differential conductance spectrum of the 1 × 1 ‐ phase (orange) compared to the DFT calculated partial density of states (PDOS) of the topmost S atoms (red). Spectrum parameters are V _stab = 2.5 V, I _stab = 0.7 nA, V _mod = 10 mV, f _mod = 811 Hz, T _s = 1.7 K.

Figure 10

DFT calculated ball model representation of the $\sqrt{3}\times \sqrt{3}$ ‐ phase with stoichiometry Nb_5/3S₃‐2D in a) top and b) side view. Nb atoms: cadet blue balls and gray; S atoms: yellow; dark gray: C atoms. c) Differential conductance spectrum of the $\sqrt{3}\times \sqrt{3}$ ‐ phase (green) compared to the DFT calculated partial density of states (PDOS) of the topmost S atoms (red). d) DFT calculated STM topograph compared to e) measurement. Spectrum parameters: V _stab = 3 V, I _stab = 0.8 nA, V _mod = 10 mV, f _mod = 797 Hz, T _s = 1.7 K.