3D Imaging and Manipulation of Subsurface Selenium Vacancies in ${\mathrm{PdSe}}_2$

Two-dimensional materials such as layered transition-metal dichalcogenides (TMDs) are ideal platforms for studying defect behaviors, an essential step towards defect engineering for novel material functions. Here, we image the 3D lattice locations of selenium-vacancy $V_{\text{Se}}$ defects and manipulate them using a scanning tunneling microscope (STM) near the surface of ${\mathrm{PdSe}}_2$, a recently discovered pentagonal layered TMD. The $V_{\text{Se}}$ show a characterisitc charging ring in a spatially resolved conductance map, based on which we can determine its subsurface lattice location precisely. Using the STM tip, not only can we reversibly switch the defect states between charge neutral and charge negative, but also trigger migrations of $V_{\text{Se}}$ defects. This allows a demonstration of direct “writing” and “erasing” of atomic defects and tracing the diffusion pathways. First-principles calculations reveal a small diffusion barrier of $V_{\text{Se}}$ in ${\mathrm{PdSe}}_2$, which is much lower than S vacancy in ${\mathrm{MoS}}_2$ or an O vacancy in ${\mathrm{TiO}}_2$. This finding opens an opportunity of defect engineering in ${\mathrm{PdSe}}_2$ for such as controlled phase transformations and resistive-switching memory device application.

Figure 1

Probing the structure and band gap on ${\text{PdSe}}_2$ surface. (a) Sketch of the crystallographic structure of ${\mathrm{PdSe}}_2$ with the purple and yellow spheres representing the Pd and Se atoms, respectively. $L{1}_a,L{1}_b,\mathrm{...}$, define Se sublayers along the [001] direction where the Se top surface layer is counted as layer $L{1}_a$. (b) A typical STM topographic image of ${\mathrm{PdSe}}_2$, blue circles indicate some $V_{\mathrm{Se}}$ defects (sample bias $V_s=-1.0\mathrm{V}$, tunneling current $I=50\mathrm{pA}$, temperature $T=120\mathrm{K}$). The inset in (b) shows the atomically resolved STM image (top right corner, Fourier filtering was applied to enhance the contrast) and simulated image (bottom right corner) of ${\mathrm{PdSe}}_2$ at 0.3 V. (c) $dI/dV$ spectrum on ${\mathrm{PdSe}}_2$ in comparison with theoretical DOS of ${\mathrm{PdSe}}_2$ ($V_s=-0.9\mathrm{V}$, $I=3\mathrm{nA}$, $V_{ac}=10\mathrm{mV}$, $f=1000\mathrm{Hz}$, $T=120\mathrm{K}$).

Figure 2

Imaging Se-vacancy defects and switching charge state of the defects in ${\text{PdSe}}_2$. (a) STM topographic image showing $V_{\mathrm{Se}}$ defects as dark disks and labeled as $V1$ to $V11$ ($V_s=0.5\mathrm{V}$, $I=50\mathrm{pA}$, $T=120\mathrm{K}$). (b)–(f) The evolution of measured differential conductance $dI/dV$ map as a function of sample bias, showing defects located on different lattice layers as rings of different diameters. (g) The dependence of ring diameter as a function of sample bias for 11 defects. (h) A sketch of the lateral tip-induced band bending on the sample below the STM tip showing the switch of charge neutral $V^0$ to negative state $V^{-}$. (i) A sketch of depth-dependent band bending of conduction band (CB), valence band (VB), and defect state ($E_D$) below the tip position.

Figure 3

Manipulation of Se-vacancy defects by a STM tip. (a),(b) STM topographic images (upper) and $dI/dV$ maps (lower) before (a) and after (b) treating the sample area with a STM tip by scanning at $V_s=-1.0\mathrm{V}$, $I=50\mathrm{pA}$. The images and $dI/dV$ maps are acquired at $V_s=0.4\mathrm{V}$, $I=50\mathrm{pA}$, $V_{ac}=50\mathrm{mV}$, $f=1000\mathrm{Hz}$, $T=120\mathrm{K}$. (c) Schematic illustrating $V_{\mathrm{Se}}$ migration to underneath the STM tip induced by the electrical field from the STM tip. (d)–(f) $V_{\mathrm{Se}}$ defects induced and removed on the surface after treating the surface by scanning at $-2$ and $+1\mathrm{V}$, respectively. The images are acquired at $V_s=0.5\mathrm{V}$, $I=50\mathrm{pA}$, $T=120\mathrm{K}$. Blue dashed circles mark the new defects; black circles indicate defect disappearance.

Figure 4

Se-vacancy diffusion between two layers or within the same layer. (a) Schematic of different $V_{\mathrm{Se}}$ configurations in the ${\mathrm{PdSe}}_2$ sample, with the red circle marking the $V_{\mathrm{Se}}$ site. (b) Calculated energy barriers for $V_{\mathrm{Se}}$ diffusion between the different defect configurations.