Influence of atomic-scale defects on coherent phonon excitations by THz near fields in an STM

TL;DR

This work demonstrates that atomic-scale defects in 2H-MoTe$_2$ modulate the coupling of THz near fields to coherent phonons, as revealed by THz-STM pump-probe measurements that detect two long-lived modes at $f_ extalpha = 0.48\,\mathrm{THz}$ and $f_ extbeta = 0.60\,\mathrm{THz}$, assigned to $E^{2}_{2g}$ and $B^{2}_{2g}$. The relative excitation of these modes is tuned by local band bending and defect-state charging, enabling defect-tunable control of phonon dynamics at the nanoscale. The study shows that defect-induced dipoles and inhomogeneous fields modify the coupling efficiency to surface phonons without shifting their frequencies, offering a pathway to engineer vibrational properties via atomic-scale defects. Overall, the work highlights how near-field THz excitation combined with defect engineering can enable selective, nanoscale manipulation of lattice dynamics with potential implications for tailoring material properties.

Abstract

Coherent phonons describe the collective, ultrafast motion of atoms and play a central role in light-induced structural dynamics. Here, we employ terahertz scanning tunneling microscopy (THz-STM) to excite and detect coherent phonons in semiconducting 2H-$MoTe_{2}$ and resolve how their excitation is influenced by atomic-scale defects. In a THz pump-probe scheme, we observe long-lived oscillatory signals that we assign to out-of-plane breathing and in-plane shear modes, which are both forbidden in the bulk. Remarkably, the relative excitation strength of these modes varies near defects, indicating that local band bending modulates the coupling to the THz field. This defect-tunable coupling offers new opportunities to control material properties via selective excitation of vibrational modes at the nanoscale.

Figures (11)

• Figure 1: STM characterization of 2H-MoTe2.(A) Sketch illustrating the coupling of THz pulses into the STM junction (not to scale). (B) Side and top view of the MoTe2 in a 2H-phase (not to scale). In-plane ($E^{2}_{2g}$) and out-of-plane ($B^{2}_{2g}$) vibrational motions are shown with red and blue arrows, respectively. (C) STM topography of the in-situ cleaved 2H-MoTe2. Tunneling parameters for recording the topography are $V_\mathrm{b}$ = 1.3 V, and $I$ = 20 pA. The scale bar is 5 nm. (D) d$I$/d$V_\mathrm{b}$ spectra recorded on the pristine surface (in black) and on two defects (in blue and green) (feedback opened at $V_\mathrm{b}$ = 0.9 V, $I$ = 20 pA and $V_\mathrm{mod}$ = 13 mV). The tip positions are marked with colored crosses (black, blue, and green) in C. Positions of the defect states ($d_\mathrm{1}$ to $d_\mathrm{5}$) for the two defects are indicated by the vertical dashed lines. The inset shows a close-up view on the negative part of the gap, bringing out the deep in-gap defect state, marked with a black arrow. All defect states are labeled $d_i$
• Figure 2: THz pulse shape in the STM junction.(A) Near-field pulse shape measured by photoemission sampling by a Ag tip in front of a 2H-MoTe2 surface. Parameters for acquiring the pulse shape, $V_\mathrm{b}$ = -5.0 V, laser repetition rate $R_\mathrm{rate}$ = 1.25 MHz. (B) Power spectral density (PSD) from Fast Fourier transform of the THz pulse shown in A. (C) THz-induced tunneling current ($I_\mathrm{THz}$) as a function of $V_\mathrm{b}$ at two different $E_\mathrm{THz}^\mathrm{inc}$ (feedback opened at $V_\mathrm{b}$ = 0.9 V and $I$ = 20 pA). The signal is integrated for 0.5 s at each data point at a pulse repetition rate $R_\mathrm{rate}$ = 10 MHz. For comparison, DC current is shown (in black) without any THz field.
• Figure 3: Terahertz pump-probe measurements on pristine 2H-MoTe2. (a) Time traces of THz-induced tunneling current ($I_\mathrm{THz}$) on the pristine region at different bias voltages (feedback opened at $V_\mathrm{b}$ = 0.9 V, $I$ = 10 pA, $R_\mathrm{rate}$ = 10 MHz). Traces are offset for clarity. (b) Power spectral density (PSD) obtained from the FFT of $I_\mathrm{THz}$ traces shown in (a). The range for the FFT is from 3 to 50 ps, which was chosen to avoid the dominant non-linearities at ultrafast timescales.
• Figure 4: Effect of defects on the phonon modes.(A) Time-resolved traces of $I_\mathrm{THz}$ recorded at different $V_\mathrm{b}$ (feedback opened at $V_\mathrm{b}$ = 0.9 V, $I$ = 30 pA, $R_\mathrm{rate}$ = 10 MHz for all traces). Measurement positions of the time traces are marked with the (red) cross in the inset topography ($V_\mathrm{b}$ = 0.9 V, $I$ = 4 pA). (B) Power spectral density (PSD) obtained from the FFT of $I_\mathrm{THz}$ traces shown in A. The range for the FFT is from 3 to 50 ps. (C) Time trace recorded on the same defect as in (A) but at closer tip sample distance, feedback opened at $V_\mathrm{b}$ = 0.6 V, $I$ = 30 pA, and (D) corresponding PSD. For all time-resolved traces a repetition rate $R_\mathrm{rate}$ = 10 MHz is used. The signal is integrated for 0.5 s at each data point. (E)Left: Energy diagram representing induced band bending at the surface of MoTe$_2$ at $V_\mathrm{b}$ > $V_\mathrm{FB}$, where $V_\mathrm{FB}$ is flat-band condition. Black/blue solid line represents the band bending on the pristine/defect area. Right: Lateral profile of the energy bands below the STM tip. Two defect states ($d_\mathrm{1}$ and $d_\mathrm{4}$) are indicated by blue dots. (F)Left: Energy diagram at $V_\mathrm{b}$ < $V_\mathrm{FB}$, and Right: Lateral profile of the energy bands below the STM tip. In both E and F, dotted black and blue line represents the chemical potential and defect state, respectively.
• Figure S1: Characterization of 2H-MoTe$_2$ sample: STM topography showing a large number of defects. Scanning parameters are $V_\mathrm{b}$ = 0.7$\,$V, and $I$ = 15 pA.