Anomalous heavy doping in chemical-vapor-deposited titanium trisulfide nanostructures

Nanoscale transition-metal trichalcogenides such as ${\mathrm{TiS}}_3$ have shown great potential for both fundamental studies and application developments, yet their bottom-up synthesis strategy is to be realized. Here we explored the chemical vapor deposition (CVD) synthesis of ${\mathrm{TiS}}_3$, whose lattice anisotropy has enabled the preferential growth along the $b$ axis, resulting in rectangular nanosheets or nanoribbons with aspect ratios tunable by the growth temperature. The obtained nanostructures, while maintaining the spectroscopic and structural characteristics as that of pristine semiconducting ${\mathrm{TiS}}_3$, exhibit high conductivities and ultralow carrier activation barriers, promising as nanoscale conductors. Our experimental and calculation results suggest that the existence of ${\mathrm{S}}_2^{2-}$ vacancies in the CVD-grown ${\mathrm{TiS}}_3$ is responsible for the heavy $n$-type doping up to a degenerate level. Moreover, the semiconducting property is predicted to be recovered by passivating the ${\mathrm{S}}_2^{2-}$ vacancies with oxygen atoms from ambient. This work hence portends the tantalizing possibility of constructing nanoscale electronics with defect-engineered trichalcogenide semiconductors.

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Figure 1

Atmospheric-pressure CVD (APCVD) growth of ${\mathrm{TiS}}_3$. (a) Experimental setup of the APCVD system. The crystal structure of ${\mathrm{TiS}}_3$ is displayed on the right. (b) Ti-S phase diagram [15]. (c) Typical OM image of ${\mathrm{TiS}}_3$ nanosheets grown at 400 °C. (d), (e) EDS mapping images of Ti and S, respectively, for a rectangular ${\mathrm{TiS}}_3$ nanosheet. (f) OM image of mixed ${\mathrm{TiS}}_3$ nanoribbons and ${\mathrm{TiS}}_2$ nanosheets grown at 450 °C. (g) Plot of average width/length (W/L) ratio of ${\mathrm{TiS}}_3$ samples vs the growth temperature. (h) AFM image and height profile of ${\mathrm{TiS}}_3$ nanoribbons.

Figure 2

Characterization of the CVD-synthesized ${\mathrm{TiS}}_3$ nanostructures. (a) Raman spectrum of ${\mathrm{TiS}}_3$. The left inset shows the OM image of the ${\mathrm{TiS}}_3$ nanoribbon being characterized. The right inset shows the intensity of the $A_{\mathrm{g}}^{\mathrm{internal}}$ peak at $371.62\mathrm{c}{\mathrm{m}}^{-1}$ as a function of the excitation polarization angle. Black dots and red lines are the experimental data of ${\mathrm{TiS}}_3$ and the ${sin}^2\theta$ function-fitted curve, respectively. (b), (c) XPS spectra of Ti and S, respectively, in the ${\mathrm{TiS}}_3$ samples. (d) TEM image (left) and selected-area electron diffraction patterns (right) of ${\mathrm{TiS}}_3$. (e) HAADF-STEM image of ${\mathrm{TiS}}_3$. The inset shows the corresponding low-magnification STEM image.

Figure 3

Electrical measurements of the CVD-synthesized ${\mathrm{TiS}}_3$ nanostructures. (a) Current-voltage curves of the ${\mathrm{TiS}}_3$ device with channel lengths varying from 0.4 to 4 μm. The inset shows the AFM image of the device. The channel width and height measured by AFM are 2.5 μm and 35.7 nm, respectively. (b) Plot of the total resistance as a function of the channel length for the device in (a). (c) Current-voltage curves of a $4\mu \mathrm{m}\times 2.5\mu \mathrm{m}\times 35.7\text{−}\mathrm{nm}$ ${\mathrm{TiS}}_3$ device under different temperatures. The inset shows the conductance change with the temperatures. (d) Arrhenius plot of the device in (c). (e) Normalized noise spectral density, $S/I^2$, as a function of frequency $f$ for ${\mathrm{TiS}}_3$ devices with different thicknesses. $\mathrm{Voltage}=0.1\mathrm{V}$. The inset shows the proportionality of S with the current at $f=10\mathrm{Hz}$. (f) Current response of a $1\mu \mathrm{m}\times 0.31\mu \mathrm{m}\times 15.6\text{−}\mathrm{nm}$ device 1#, $0.9\mu \mathrm{m}\times 2.5\mu \mathrm{m}\times 35.7\text{−}\mathrm{nm}$ device 2#, and $0.4\mu \mathrm{m}\times 2.5\mu \mathrm{m}\times 35.7\text{−}\mathrm{nm}$ device 3# to the electrical field.

Figure 4

Theoretical understanding of the vacancy-induced heavy doping in CVD-grown ${\mathrm{TiS}}_3$. Shown in the upper row are top-view $(3\times 3)$ supercells of (a) intrinsic monolayer ${\mathrm{TiS}}_3$, (c) monolayer ${\mathrm{TiS}}_3$ with ${\mathrm{S}}_2^{2-}$ vacancies, and (e) monolayer ${\mathrm{TiS}}_3$ with the ${\mathrm{S}}_2^{2-}$ vacancies filled by oxygen pairs. For better visualization, S atoms in different layers are presented in different colors. Purple and blue atoms illustrate ${\mathrm{S}}_2^{2-}$. Yellow and green atoms illustrate ${\mathrm{S}}^{2-}$. Gray atoms illustrate ${\mathrm{Ti}}^{4+}$. Red atoms illustrate ${\mathrm{O}}_2^{2-}$. Dashed circles mark the position of ${\mathrm{S}}_2^{2-}$ vacancies. Corresponding PDOS spectra are presented in (b), (d), and (f), respectively, in the lower row. Dashed lines indicate where the Fermi levels are.