Temperature-sensitive spatial distribution of defects in $\mathrm{Pd}{\mathrm{Se}}_2$ flakes

Defect engineering plays an important role in tailoring the electronic transport properties of van der Waals materials. However, it is usually achieved through tuning the type and concentration of defects, rather than dynamically reconfiguring their spatial distribution. Here, we report temperature-sensitive spatial redistribution of defects in $\mathrm{Pd}{\mathrm{Se}}_2$ thin flakes through scanning tunneling microscopy. We observe that the spatial distribution of Se vacancies in $\mathrm{Pd}{\mathrm{Se}}_2$ flakes exhibits a strong anisotropic characteristic at 80 K, and that this orientation-dependent feature is weakened when temperature is raised. Moreover, we carry out transport measurements on $\mathrm{Pd}{\mathrm{Se}}_2$ thin flakes and show that the anisotropic features of carrier mobility and phase coherent length are also sensitive to temperature. Combining with theoretical analysis, we conclude that temperature-driven defect spatial redistribution could interpret the temperature-sensitive electrical transport behaviors in $\mathrm{Pd}{\mathrm{Se}}_2$ thin flakes. Our work highlights that engineering spatial distribution of defects in the van der Waals materials, which has been overlooked before, may open up an avenue to tailor the physical properties of materials and explore different device functionalities.

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Figure 1

(a) Crystallographic structures of $\mathrm{Pd}{\mathrm{Se}}_2$. Top panel: Schematics of three-dimensional $\mathrm{Pd}{\mathrm{Se}}_2$ lattice structure. Bottom panel: the top view. (b) The photograph of $\mathrm{Pd}{\mathrm{Se}}_2$ single crystals grown by self-flux method. (c) X-ray diffraction (XRD) pattern. (d) Energy dispersive x-ray spectroscopy (EDXS). Inset: The SEM image of one typical $\mathrm{Pd}{\mathrm{Se}}_2$ single crystal. (e) The Raman spectra of $\mathrm{Pd}{\mathrm{Se}}_2$ flakes. Inset: the fitted peak intensities of $A_{\mathrm{g}}^1\text{−}{B}_{1\mathrm{g}}^1$ mode vs sample rotation angle θ under parallel-polarization configuration. (f) High-resolution TEM image. Inset: the optical microscopy image of one $\mathrm{Pd}{\mathrm{Se}}_2$ flake on the carbon-coated copper mesh (upper left). Selected-area electron diffraction (SAED) pattern of the $\mathrm{Pd}{\mathrm{Se}}_2$ flake (bottom left).

Figure 2

Characterization of Se-vacancy defects by STM tip. (a) A STM topographic image of $\mathrm{Pd}{\mathrm{Se}}_2$, where Se vacancy ($V_{\mathrm{Se}}$) defects are marked with white circles ($V\mathrm{s}=-190\mathrm{mV}, I=300\mathrm{pA}$). Upper right: sketch of the crystallographic structure of $\mathrm{Pd}{\mathrm{Se}}_2$; lower right: atomically resolved STM image of $\mathrm{Pd}{\mathrm{Se}}_2$. (b),(c) STM topographic images and the statistical histogram of $V_{\mathrm{Se}}$ distribution along $a$ and $b$ axes at 80 and 110 K, respectively ($b:V\mathrm{s}=-800\mathrm{mV}, I=150\mathrm{pA}$; $c:V\mathrm{s}=-500\mathrm{mV}, I=150\mathrm{pA}$).

Figure 3

(a) Schematic geometry of $\mathrm{Pd}{\mathrm{Se}}_2$ device and the transport measurement setup. (b),(c) Gate dependent conductivity σ from 250 to 1.6 K along $a$ and $b$ axes, respectively. Inset: optical image of $\mathrm{Pd}{\mathrm{Se}}_2$ device. For clear distinction, the conductivity value of the curve between adjacent temperatures is offset by 10 μS from 250 to 1.6 K. (d) The conductivities along $a$ and $b$ axes vs temperature, with the gate voltage at 70 V. (e) Mobility along $a$ and $b$ axes, and their ratio vs temperature at a given gate voltage 70 V.

Figure 4

Anisotropic quantum interference effects in $\mathrm{Pd}{\mathrm{Se}}_2$. (a),(b) The gate dependent magnetoconductivity at 1.6 K with current along $a$ and $b$ axis, respectively. The solid lines are the fitting curves of the magnetoconductivity data with HLN model. (c) The fitted phase coherence lengths $L_{\phi }$ vs $V_{\mathrm{bg}}$ at 1.6 K. (d) The fitted $L_{\phi }$ vs $T$ at $V_{\mathrm{bg}}=80\mathrm{V}$.