Open Access
Add to BibSonomy
Abstract
Terahertz emission by ultrafast excitation of semiconductor/metal interfaces was found strongly enhanced by plasmon resonance. Here, a three-dimensional nanoporous gold (NPG) was used to form semiconductor/metal compound with cadmium telluride (CdTe). We investigated the specific impact of surface plasmon from randomly nanoporous structure in the ultrafast optoelectronic response for THz generation, and observed a THz amplitude enhancement around an order of magnitude from CdTe on NPG compared to that from CdTe on silicon. Moreover, the plasmon enhancement for THz emission from NPG is stronger than that from gold film, indicating that randomly nanoporous structure is also effective for plasmonic enhancement in THz band.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The explosively growing investigations of mixed dimensional heterostructures suggest new avenues for the designation of high-performance optoelectronic devices [1–4]. Integration of noble metal nanostructures with semiconductors has demonstrated remarkable advantage due to the affected optoelectronic response from the hot electrons in plasmonic nanostructures through electromagnetic decay of surface plasmons [5,6]. Large optical absorption and fast charge transport have been demonstrated as the fundamental physics processes for photovoltaic and photocatalytic devices base on semiconductor/metal heterostructures.
Terahertz science and technology rapidly expand in the past 20 years due to the unique applications in material characterization [7], nondestructive detection [8], high speed communication [9], and high qualified THz information depends on efficient terahertz sources and detectors [10–12]. The development of ultra-fast optoelectronic technology and low-scale semiconductor technology provide appropriate light source and detection means for THz band [13–16], and semiconductor compound devices is one promising choice to realize detectors and emitters operating in THz together.
Since 1990s, compound semiconductors [17,18] have been studied extensively for THz emitters based on the photo-generated emission. Built-in electric field induced transient drift current in GaAs and photo-Dember field induced transient photocurrent in InAs are two of the most efficient methods for THz generation at low pump fluence. And the THz emission can be further improved by carrier doping [19], surface modification [20], applied electric field [21] and surface plasmon modulation. Generally, THz emission with ultrafast laser pump is related to the time derivative of the photo-generated current. Surface plasmon has been demonstrated with efficient enhancement of photo-conductive, which could furtherly enhance the THz emission. Y. M. Bahk's research shows that surface plasmons, excited with femtosecond laser pulses on continuous or discontinuous gold substrates, strongly enhance the emission of ultrashort, broadband terahertz pulses from single layer graphene [22]. Bhattacharya's work shows that enhanced terahertz emission bandwidth from photoconductive antenna by manipulating carrier dynamics of semiconducting substrate with embedded plasmonic metasurface [23]. And G. Ramakrishnan's research reports on the strong enhancement of the THz emission after optical excitation of surface plasmons at semiconductor/metal interfaces [5]. These heterostructure interfacial devices utilize the tunable Fermi level and charge transport at the interface, at which build-in electric field and carrier dynamics can be tuned efficiently.
In this study, we constructed Cadmium telluride on three-dimensional nanoporous gold (CdTe@NPG), and obtained over ten times enhanced THz radiation comparing with CdTe on silicon. We systematically investigate the ultrafast transient terahertz emission excited by femtosecond laser pulses. As an excellent substrate of surface plasmon, NPG provides efficiently enhanced carrier dynamics, resulting in an efficient THz generation from CdTe on it, which lead to an extension of fabricating cheaper and compact THz sources.
2. Materials and methods
Cadmium telluride (CdTe) was chosen to form semiconductor/NPG conpound because it is a typical wide band-gap semiconductor for THz generation [24]. CdTe thin films were grown on low resistance (100) silicon (ρ<100 Ω/cm-1), gold substrates and nanoporous gold (NPG). The CdTe thin films were fabricated using an evaporation deposition system. CdTe powder (Leshan Kai Yada Photoelectric Technology Co., Ltd; 99.999% purity) was evaporated from a tungsten wire heated quartz boat. The thickness was monitored by a quart crystal monitor with the deposition rate of 1∼3 Å/s. During the deposition, the substrate was kept rotating to improve the lateral uniformity of the film across the substrate. The NPG substrates were obtained by de-alloying a precursor Au25Ag75 (wt. %) sputtered on silicon wafer with a chromium layer and a gold layer for buffering, using 71% nitric acid (Sinopharm Chemical Reagent) at room temperature for 5 minutes. both the planar gold substrates and Au25Ag75 (wt. %) precursor substrates with the thickness of ∼205 nm (see fig. S1) were sputtered by a magnetron sputtering instrument on low resistance (100) silicon (ρ<100 Ω/cm-1). All the metal targets are provided by Zhongnuo New Materials (Beijing) Technology Company.
The schematic diagram of the experimental setup is shown in Fig. 1(a). To emphasize the role played by plasmon in the generation of THz light, an optical fiber laser was employed to provide 780 nm laser pulses with 150 fs pulse duration and 80 MHz repetition rate. The output laser through a beam-splitter is divided into a strong part as the pump beam and a weak part as the probe beam. The pump beam passes through a time delay device and then irradiates onto the front surface of the sample with 45° incident angle, and then the generated terahertz pulses are detected at a reflection emission measurement mode [25,26] [see Fig. 1(b)]. A half wave plate (HWP) is used to change the polarization state of the pump beam. To improve the power density, the pump beam is focalized to the spot area of ∼50 μm2. The THz radiation pulses are collected by two off-axis parabolic mirrors and then focused onto a photoconductive antenna (PCA, BATOP GmbH) that pumped by probe beam. Before the PCA, a Teflon wafer is used to prevent the redundant 780 nm pump laser, and the final THz signal is collected by the phase-locked amplifier (Stanford SR830). All the measurements are carried out in a super clean room with the relative humidity 60% and temperature 24 ℃.
Fig. 1. (a) Experimental setup of the terahertz time-domain emission spectroscopy; RM: Reflective mirror; HWP: half-wave plate; OAP: off-axis parabolic mirror; PCA: photoconductive antenna; TF: Teflon wafer. (b) Schematic diagram of the femtosecond (fs) laser pulses induce terahertz (THz) emission from CdTe@NPG sample; θ represents the incident angle; α represents the azimuth angle.
3. Results and discussion
Scanning electronic microscope (SEM) and transmission electron microscope (TEM) images of CdTe@NPG are shown in Fig. 2. The nanoporous gold uniformly distributed on the surface of wafer with constant cracks which caused by residual stress releasing during dealloying. From TEM images [Fig. 2(c)-2(e)] it can be seen that the 10 to 15 nm thick CdTe layer is mainly amorphous and embed several nanocrystal with 3-5 nm diameter.
Fig. 2. (a∼b) SEM of CdTe@NPG; (c∼e) TEM of CdTe@NPG, CdTe nanocrystal is highlighted by yellow dashed line.
To further confirm the morphology and compose of CdTe@NPG, X-Ray diffraction (XRD) pattern and Raman spectrum are presented in Fig. 3. Figure 3(a) shows the Raman spectrum from CdTe@NPG measured at room temperature. Four characteristic peaks can be seen from the Raman spectrum of CdTe@NPG, and E mode of Te at 88 cm-1, A1 mode of Te 117.2 cm-1, E mode of Te or TO mode of CdTe at 134.6 cm-1 and LO mode of CdTe at 158 cm-1 are corresponding to CdTe crystal [27]. A broad band can be observed from the XRD spectrum of CdTe@Si film [Fig. 3(b)], indicating the majority of CdTe layer is amorphous, and the existence of two small peaks located at 23.76 and 39.31 that related with cubic CdTe provides the further evidence of partial crystallization of the thin film. The absorption spectra of NPG and CdTe@NPG (Fig. S2) shows that the CdTe coating caused redshift and broaden of resonance absorbance from localized surface plasmon resonance, and 780 nm laser excitation is included in this region.
Fig. 3. (a) Raman spectroscopy of CdTe@NPG, the wavelength of excitation of the laser is 532 nm and the power is 10 mW; (b) XRD of CdTe@Si.
THz emission spectra of CdTe film on different substrates were measured and shown in Fig. 4(a). By comparing the signal intensity, we found that the THz emission amplitude on NPG is about 10 times stronger than that on Si and about 7 times stronger than that on flat gold. The difference of the emission intensity from CdTe film on gold and NPG indicates that NPG film with structure in nano scale can provide more enhancement of THz emission which is related with the intensity of the local surface plasmon.
Fig. 4. (a) THz emission spectrum of CdTe film on different substrate; (b) Schematic energy band diagram of CdTe@NPG junction and the surface emission field after optic illumination with 780 nm.
Since THz radiation is proportional to the time derivative of the photocurrents generated by the ultrafast intensive laser pulses and nonlinear polarization depended by material, which can be described as follows:
When an incoming photon energy is above the bandgap energy of the material, the absorption of the photon will cause an electron make a transition from the valence band to the conduction band and for a pair of carriers, the photo-generated carriers will be accelerated by intrinsic electric fields, and then induce THz radiation. Surface depletion field is one of the intrinsic fields induced by the band bending at the surface. Both conduction and valence bands bend due to the Fermi level difference between surface and inferior region of semiconductor, and this surface field will introduce the photocurrent surge. When CdTe is modified on NPG, these two materials form a metallic semiconductor contact. When the laser irradiates on the sample, laser passes through the CdTe thin film to the NPG substrate. Electrons in gold are excited and accumulated to form localized surface plasmon resonance (LSPR) field. LSPR can occur in properly designed nanostructures in which confined free electrons oscillate with the same frequency as the incident radiation and eventually enter resonance, giving rise to intense, highly localized electromagnetic fields [6]. Similarly to the photo-Dember effect, the plasmon field induced by charges accumulation in NPG will produce diffusion photocurrent from NPG to CdTe [Fig. 1(b)]. This diffusion photocurrent participates into the carrier dynamics in CdTe, furtherly improve the ETHz according to the Eq. (1). In the point of semiconductor physics, this structure is equivalent to a Schottky section. Due to the so called ‘Dember field’ as show in Fig. 4(b), both conduction and valence bands bend downwards, which means that the electronic potential difference for the electrons of across the barrier increases [29]. Together with the strong LSPR from NPG surface which may affects the electron mobility and local density, the photocurrent density $\vec{J}$ from the metal to the semiconductor is further improved.
To verify the enhanced photocurrent discussed above, we tested the i-v and i-t characteristic curve of CdTe@NPG sample by four-probe method with a Keithley 2450 SourceMeter (see Ref. 30). Under the condition of thermal ionization emission, the i-v characteristic of metal semiconductor contact can be expressed as follows:
where ${J_s}$ is the saturation current density, kT is the thermal energy, q is the electron charge, V is the voltage [29]. As shown in Fig. 5(a), the current increases exponentially with voltage at low voltage (insert figure) and the slope of curve gradually stabilizes showing in line at higher voltage, which fitted well with Eq. (2). The i-v characteristic curve is same as Schottky section which confirmed the interface type between CdTe film and NPG. And the Fig. 5(b) shows the dependence of current on the switching on/off the laser. The detected current with laser on CdTe@NPG is more obvious comparing with that on CdTe@Si, indicating NPG can seriously improve the intensity of the photocurrent, and play a key role to the enhancement of THz.Fig. 5. (a) Current versus voltage characteristic curve of CdTe@NPG; (b) current versus time characteristic curve of CdTe@NPG
When femtosecond laser pulses illuminate onto the CdTe@NPG surface at oblique incidence, in principle, almost all the ultrafast photocurrents, including nonlinear current, drift current and diffusion current [31], contribute to the terahertz radiation and reflected in the intensity of detected terahertz pulses, and the photocurrents can be written as the sum of drift current Jdri, diffusion current Jdif and nonlinear current Jnl,
In order to further investigate the specific contribution of different photocurrents to the detected terahertz pulses, we measured the THz emission spectra changing the polarization and power of the excitation laser. The Jnl in this study should be corresponded to the electric field induced optical rectification (EFIOR) which depends on the pump polarization [32]. However surface depletion field induced transient photocurrent Jdri and the diffusion currents due to the photo-Dember effect Jdif are independent of pump polarization. To distinguish the contribution from Jnl and Jdif/Jdri, we test the relationship between THz amplitude and pump polarization. The relationship between THz amplitude and pump polarization is shown in Fig. 6(a). We fit the pump polarization dependent THz amplitude by ${E_{THz}} = {C_{current}} + {C_{EFIOR}}cos2\alpha $, where ${C_{current}}$ corresponds to the transient photocurrent effect, ${C_{EFIOR}}$ refers to the magnitude of optical rectification. The fitting results show about 23 times larger contribution to THz emission from transient photocurrent than from EFIOR effect, which suggest the transient photocurrent dominate the THz emission from CdTe@NPG. In the following experiment, THz radiation would be detected in an optimized condition.Fig. 6. (a) Pump polarization dependent THz amplitude; (b) THz emission amplitude from CdTe@NPG with respect to the pump power; THz emission spectrum of CdTe@NPG with (c) 45° incident angle and (d) 67.5° incident angle.
The drift currents are generated from surface-depletion field Es, which is a static electric field caused by Fermi-level pinning at the surface [33], which can be described as follows:
where N is the density of the photogenerated carrier, e is the electron charge, and μ is the mobility. The surface-depletion field Es is irrelevant to pump power Iphoton, while the density of photogenerated carrier N is proportional to Iphoton. The diffusion currents are caused by the photo-Dember effect. The photoexcited electron-hole pairs form a density gradient along the longitudinal direction. The mobility difference between the electrons and holes leads to outward diffusion currents [33], which can be written as follows: where z is the longitudinal direction. The photo-Dember electric field Ed is proportional to the photo-generated carriers density gradient. The diffusion currents are proportional to the multiple of the transient electric field and the excited carrier density and thus proportional to the square of pump power. Experimental results of terahertz signals in relationship to pump power in Fig. 6(b) fit well under the quadratic function in the combination of linear part and nonlinear part.Meanwhile, to distinguish the drift current Jdri and the diffusion current Jdif, we test the THz amplitude at another incident angle of 67.5° because the photo-Dember electric field Ed is perpendicular to the sample surface and the surface-depletion field Es is parallel to the sample surface. On account of collection efficiency variation leading by the change of experimental setup, we chose a piece of intrinsic (100) GaAs (∼1.05 Ω·cm, ∼5000 cm2/v.s) sample with 360 μm thickness as a reference. For THz emission from intrinsic GaAs, the drift current dominates the process [7]. As shown in the Figs. 6(c) and 6(d), when the incident angle is 45°, the THz signal amplitude of CdTe@NPG is similar to that of GaAs. While the incident angle turned to 67.5°, the THz signal amplitude of CdTe@NPG became larger than that of GaAs. If we simply consider as ${E_{THz}} = {C_{{E_d}}}\cdot \cos ({90 - \theta } )+ {C_{{E_s}}}\cdot \sin ({90 - \theta } )$, where ${C_{{E_d}}}$ corresponds to the photo-Dember electric field effect, ${C_{{E_s}}}$ refers to the surface-depletion field effect. The fitting results show about 2 times larger contribution to THz emission from photo-Dember electric field effect than from surface-depletion field effect. During the emission of THz from CdTe@NPG, ‘photo-Dember’ electric field caused by LSPR makes a greater contribution to amplitude of THz. In other words, the enhancement of THz signal could be attributed to the ‘photo-Dember’ electric field caused by LSPR which excited from NPG.
4. Conclusion
In summary, we have presented an effective surface enhanced THz generation by constructing CdTe on three-dimensional NPG compound structure. By varying the substrates of CdTe films, we have demonstrated that the strongly localized surface plasmon of NPG film play a major role in enhancing of the THz emission. Based on our experimental results, we establish a model to depict the ultrafast carrier dynamics. Under the pump laser, the plasmon field due to the concentration of laser induced charges in NPG bends downwards the conduction and valence bands of CdTe, which introduce the photocurrent from NPG to CdTe enhancing the photocurrent density in CdTe, furtherly enhanced the THz generation. The THz emission on CdTe@NPG has a more than ten times enhancement compared to the THz emission on CdTe@Si. Our investigations not only reveal an efficient method for strongly enhanced THz generation, but also could help to understand the role of photoelectrons excited from NPG in THz generation. Furthermore, surface plasmon enhancement terahertz emission will pave the way to cheaper and compact THz sources in the future.
Funding
National Natural Science Foundation of China (61875126, 11727812, 61927813).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. D. Jariwala, T. Marks, and M. Hersam, “Mixed-dimensional van der Waals heterostructures,” Nat. Mater. 16(2), 170–181 (2017). [CrossRef]
2. W. Yu, Z. L. H. Zhou, Y. Chen, Y. Wang, Y. Huang, and X. Duan, “Vertically stacked multi-heterostructures of layered materials for logic transistors and complementary inverters,” Nat. Mater. 12(3), 246–252 (2013). [CrossRef]
3. I. V. Grigorieva and A. K. Geim, “Van der Waals heterostructures,” Nature 499(7459), 419–425 (2013). [CrossRef]
4. H. Wang, H. Yuan, S. Sae Hong, Y. Li, and Y. Cui, “Physical and chemical tuning of two-dimensional transition metal dichalcogenides,” Chem. Soc. Rev. 44(9), 2664–2680 (2015). [CrossRef]
5. G. Ramakrishnan, N. Kumar, G. K. P. Ramanandan, A. J. L. Adam, R. W. A. Hendrikx, and P. C. M. Planken, “Plasmon-enhanced terahertz emission from a semiconductor/metal interface,” Appl. Phys. Lett. 104(7), 071104 (2014). [CrossRef]
6. C. Clavero, “Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices,” Nat. Photonics 8(2), 95–103 (2014). [CrossRef]
7. Y. Huang, Z. Yao, C. He, L. Zhu, L. Zhang, J. Bai, and X. Xu, “Terahertz surface and interface emission spectroscopy for advanced materials,” J. Phys.: Condens. Matter 31(15), 153001 (2019). [CrossRef]
8. M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1(2), 97–105 (2007). [CrossRef]
9. T. Nagatsuma, G. Ducournau, and C. C. Renaud, “Advances in terahertz communications accelerated by photonics,” Nat. Photonics 10(6), 371–379 (2016). [CrossRef]
10. R. Koehler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. Giles Davies, D. A. Ritchie, R. C. Lotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417(6885), 156–159 (2002). [CrossRef]
11. B. A. Knyazev, G. N. Kulipanov, and N. A. Vinokurov, “Novosibirsk terahertz free electron laser: instrumentation development and experimental achievements,” Meas. Sci. Technol. 21(5), 054017 (2010). [CrossRef]
12. H. Y. Hwang, S. Fleischer, N. C. Brandt, and B. G. Perkins Jr., M. Liu, K. Fan, A. Sternbach, X. Zhang, R. D. Averitt, and K. A. Nelson, “A review of non-linear terahertz spectroscopy with ultrashort tabletop-laser pulses,” J. Mod. Opt. 62(18), 1447–1479 (2015). [CrossRef]
13. R. Gordon, “Nanostructured metals for light-based technologies,” Nanotechnology 30(21), 212001 (2019). [CrossRef]
14. W. Xu, L. Xie, and Y. Ying, “Mechanisms and applications of terahertz metamaterial sensing: a review,” Nanoscale 9(37), 13864–13878 (2017). [CrossRef]
15. P.U. Jepsen, D.G. Cooke, and M. Koch, “Terahertz spectroscopy and imaging - Modern techniques and applications,” Laser & Photon. Rev. 5(1), 124–166 (2011). [CrossRef]
16. S. S. Dhillon, M. S. Vitiello, E. H. Linfield, A. G. Davies, M. C. Hoffmann, J. Booske, C. Paoloni, M. Gensch, P. Weightman, G. P. Williams, E. Castro-Camus, D. R. S. Cumming, F. Simoens, I. Escorcia-Carranza, J. Grant, S. Lucyszyn, M. Kuwata-Gonokami, K. Konishi, M. Koch, C. A. Schmuttenmaer, T. L. Cocker, R. Huber, A. G. Markelz, Z. D. Taylor, V. P. Wallace, J. A. Zeitler, J. Sibik, T. M. Korter, B. Ellison, S. Rea, P. Goldsmith, K. B. Cooper, R. Appleby, D. Pardo, P. G. Huggard, V. Krozer, H. Shams, M. Fice, C. Renaud, A. Seeds, A. Stöhr, M. Naftaly, N. Ridler, R. Clarke, J. E. Cunningham, and M. B. Johnston, “The 2017 terahertz science and technology roadmap,” J. Phys. D: Appl. Phys. 50(4), 043001 (2017). [CrossRef]
17. B. B. Hu, X. Zhang, and D. H. Auston, “Terahertz radiation induced by subband-gap femtosecond optical excitation of GaAs,” Phys. Rev. Lett. 67(19), 2709–2712 (1991). [CrossRef]
18. P. Gu, M. Tani, S. Kono, K. Sakai, and X. Zhang, “Study of terahertz radiation from InAs and InSb,” J. Appl. Phys. 91(9), 5533–5537 (2002). [CrossRef]
19. K. Liu, A. Krotkus, K. Bertulis, J. Xu, and X. Zhang, “Terahertz radiation from n-type GaAs with Be-doped low-temperature-grown GaAs surface layers,” J. Appl. Phys. 94(5), 3651–3653 (2003). [CrossRef]
20. X. Wu, B. Quan, X. Xu, F. Hu, X. Lu, C. Gu, and W. Li, “Effect of inhomogeneity and plasmons on terahertz radiation from GaAs (1 0 0) surface coated with rough Au film,” Appl. Surf. Sci. 285, 853–857 (2013). [CrossRef]
21. L. Xu, X. Zhang, D. H. Auston, and B. Jalali, “Terahertz radiation from large aperture Si p-i-n diodes,” Appl. Phys. Lett. 59(26), 3357–3359 (1991). [CrossRef]
22. Y. M. Bahk, G. Ramakrishnan, J. Choi, H. Song, G. Choi G, Y. H. Kim, K. J. Ahn, D. S. Kim, and P. C. M. Planken, “Plasmon enhanced terahertz emission from single layer graphene,” ACS Nano 8(9), 9089–9096 (2014). [CrossRef]
23. A. Bhattacharya, D. Ghindani, and S. Prabhu, “Enhanced terahertz emission bandwidth from photoconductive antenna by manipulating carrier dynamics of semiconducting substrate with embedded plasmonic metasurface,” Opt. Express 27(21), 30272–30279 (2019). [CrossRef]
24. Z. Chu, J. Liu, and J. Liu, “Study of THz-wave-induced photoluminescence quenching in GaAs and CdTe,” Appl. Phys. B 109(1), 113–119 (2012). [CrossRef]
25. M. B. Johnston, D. M. Whittaker, A. Corchia, A. G. Davies, and E. H. Linfield, “Simulation of terahertz generation at semiconductor surfaces,” Phys. Rev. B 65(16), 165301 (2002). [CrossRef]
26. J. Han, Y. Choi, and J. Lee, “Radiating pattern of surge-current-induced THz light in near-field and far-field zone,” Sci Rep 8(1), 6513 (2018). [CrossRef]
27. D. Mohanty, Z. Lu, X. Sun, Y. Xiang, L. Gao, J. Shi, L Zhang, K. Kisslinger, M. A. Washington, and G. Wang, “Growth of epitaxial CdTe thin films on amorphous substrates using single crystal graphene buffer,” Carbon 144, 519–524 (2019). [CrossRef]
28. G. K. Kitaeva, “Terahertz generation by means of optical lasers,” Laser Phys. Lett. 5(8), 559–576 (2008). [CrossRef]
29. S. M. Sze and K. K. Ng, Physics of Semiconductor Device, 3rd Ed (Wiley Interscience, 2007).
30. A. A. Meier and D. I. Levinzon, “Application of the four-probe method for measuring the resistivity of nonuniform semiconductor materials,” Meas. Tech. 8(5), 427–429 (1965). [CrossRef]
31. Z. Fang, H. Wang, X. Wu, S. Shan, and L. Wang, “Nonlinear terahertz emission in the three-dimensional topological insulator Bi2Te3 by terahertz emission spectroscopy,” Appl. Phys. Lett. 115(19), 191102 (2019). [CrossRef]
32. Z. Yao, Y. Huang, L. Zhu, P. A. Obraztsov, W. Du, L. Zhang, and X. Xu, “Interfacial THz generation from graphene/Si mixed-dimensional van der Waals heterostructure,” Nanoscale 11(35), 16614–16620 (2019). [CrossRef]
33. D. Halliday, R. Resnick, and K. Krane, Physics (John Wiley & Sons, 2002).
