Regulating Terahertz Photoconductivity in Two-Dimensional Materials

Abstract

Two-dimensional materials represented by graphene have attracted extensive interest owing to the unique layer-dependent physical properties that are tunable with various external fields. In addition, by stacking two or more 2D materials together, a new material with the desired properties can be tailored and designed. Fully understanding the dynamical photoconductive response in 2D materials is uttermost important to design and develop the advanced optoelectronic devices. Terahertz (THz) time-domain spectroscopy (TDS) and time-resolved THz spectroscopy are powerful spectroscopic tools with the advantages of being contact-free and noninvasive, which have been widely used to study the photoconductivity (PC) of 2D materials. In this review, firstly, we provide a short introduction of the 2D materials and THz spectroscopy, and then a brief introduction of the experimental setup and experimental data analysis based on time-resolved THz spectroscopy are presented. After that, we overview the latest progress on the regulation of the THz PC that includes: (1) regulating the THz PC of graphene (Gr) and transition metal dichalcogenide (TMD) thin films with oxygen adsorption; (2) regulating the THz PC of Gr and Gr/TMDs heterostructures by electric gating and a built-in field introduced by a substrate; (3) regulating the THz PC of Gr/TMD heterostructures via optical gating; and (4) we overview the latest progress on the observation of elementary excitations in 2D materials with THz PC spectra following optical excitation and THz PC regulation via the photoexcitation of quasi-particles. Finally, we conclude the review and present a short overview of future research directions.

1. Introduction

Since the discovery of single-layer graphene in 2004 [1], the emerging two-dimensional materials have fundamentally changed the landscape of materials science [2,3,4,5]. Owing to its unique properties, high stability in air, high tunability, and easy availability, studies on 2D materials have increased in the last 20 years [1,2,3,4,5,6,7,8]. Two-dimensional materials consist of a wide range of materials, in which the constituent atoms are strongly coupled with covalent bonds in plane and weakly coupled through van der Waals (vdW) interactions in the out-of-plane direction. In addition to the main material of graphene (Gr), the most studied 2D materials also include transition metal dichalcogenides (TMDs) [9], hexagonal boron nitride (hBN) [10], silicene (2D silicon) [11], MXene (2D carbide/nitrides) [12], phosphorene [13], and borophene (2D boron) [14], as well as the heterostructure constructed by stacking two or more monolayer 2D materials together with a particular stacking sequence [3,7,15,16]. Most 2D materials have a hexagonal structure; thus, it is possible to stack them with other 2D materials to enrich their properties.

Two-dimensional materials provide a powerful platform for exploring fundamental science and have several potential applications. Moreover, tunability and comparability make 2D materials and their heterostructures promising candidates for finding new materials and designing advanced devices in the field of electronics, optoelectronics, and photonics, such as photo-transistors [17,18], solar cells [19], light-emitting diodes [20], photodetectors [21], modulators, switches, and flexible electronics [22]. In a 2D material, the (photo-)carriers are confined in a two-dimensional space that results in interesting properties that the bulk counterpart does not have, such as large exciton binding energy, vast enhancement of optical nonlinearity, rich quantum freedoms (for instance, spin, psedospin, and valleytronic), and novel states of matter (e.g., Ising superconductivity and 2D magnetism). These quantum degrees of freedom as well as their coupling with light, electric, and magnetic fields have initiated novel physical phenomena in 2D materials. In addition, 2D heterostructures constructed with 2D materials are no longer limited by lattice-matching conditions, an atomic interdiffusion that was required in traditional heterostructures, thus providing a unique platform for the development of optoelectronic devices. The application of 2D materials in various electronic and photonic devices requires an in-depth understanding of carrier dynamics in both equilibrium and non-equilibrium states. THz spectroscopy has been demonstrated to be a powerful spectroscopic tool to explore the static and non-equilibrium state of carriers in a contactless way.

Briefly, THz radiation refers to the electromagnetic wave band with frequency that ranges from 0.1 THz to 10 THz, which corresponds to the photoenergy range from 0.4 meV to 40 meV and a wavelength range from 3 mm to 30 μm [23,24,25,26,27]. The THz band falls between low-frequency electronics and high-frequency photonics: Considering the working frequency of gigahertz in most modern microelectronic devices, it is desired to develop high-speed electronic device working in the THz frequency. On the other hand, comparing the low photon energy to visible light, THz radiation is very sensitive to the free carrier around the Fermi surface, which makes THz spectroscopy a crucial tool for accessing carrier dynamics in addition to transient absorption spectroscopy. THz time-domain spectroscopy (TDS) can provide the real and imaginary parts of complex conductivity simultaneously without referring to Kramers–Kronig relation. Optical pump and THz probe (OPTP) spectroscopy is a powerful tool to explore the photocarrier dynamics of condensed matters that has been applied to access the dynamical THz PC in 2D materials [24,27]. The OPTP can directly access the relaxation dynamics of photoexcited carriers, including radiation and non-radiation recombination. As a low photon energy spectroscopy with sub-ps time resolution, OPTP is a unique tool to assess the intraband scattering by analyzing time-dependent complex PC, through which a clear and conclusive carrier transport picture can be obtained [27]. In addition, different materials show distinct spectral signatures that are relevant to the pump fluence, wavelength, and temperature, as well as delay time between pump and THz pulses, and the complex THz PC can be used to address fundamental questions, such as dynamical charger transfer and subsequent relaxation, formation and relaxation of low-energy elementary excitation, and the mode hybridization of different collective excitations. From an application point of view, the dynamic THz PC also can provide an alternative way to design high-speed modulators, switches, as well as photo-transistors.

In this article, we provide a comprehensive review of the investigations into regulating THz PC in ultrathin 2D-layered materials as well as their heterostructures. Following this brief introduction, we provide a short description of the experimental setup, including THz time-domain spectroscopy and time-resolved THz spectroscopy, as well as the physical models used to analyze the experimental data. Then, we overview the latest progress on regulating THz PC in 2D materials from four perspectives: In Section 3, we review the influence of gas absorption on THz PC in graphene and TMDs films, in which we mainly discuss the effect of oxygen absorption on the thin films’ PC. In Section 4, we review the impact of electric gating on THz PC in graphene as well as the tuning of THz PC in Gr/TMD heterostructures by a built-in field introduced by a substrate. In Section 5, we provide an overview on the regulation of THz PC in Gr/TMD heterostructures via photogating. In Section 6, we discuss the regulation of THz PC via the photoinduced formation of novel elementary excitations, such as trion, small polaron, and large polaron. Finally, we conclude the review with a discussion on the potential applications of 2D materials based on PC, and a short outlook is also presented discussing the rapidly developing research in this field.

2. THz Experimental Setup and THz Data Analysis

A typical optical pump and THz probe (OPTP) spectroscopy are displayed schematically in Figure 1 [28,29]. An ultrafast laser pulse train is delivered from a femtosecond laser with a typical pulse duration of ~100 fs and pulse energy of a few mJ. The BS1 is used to split the laser beam into two beams, the reflection beam passes through an optical parametric amplifier (OPA), and the output wavelength from OPA is tunable from ultraviolet to visible to near and middle infrared that acts as a pump beam. The BS2 is used to further split the transmitted beam into the THz generation beam and the THz detection beam. The reflection beam from BS2 is focused on a (110)-oriented ZnTe crystal to generate a THz pulse via optical rectification. The generated THz radiation is then collimated and focused by a pair of off-axis parabolic mirrors PM1 and PM2 on the surface of the sample. The THz pulse through the sample is collimated and focused by another pair of parabolic mirrors PM3 and PM4 on the another (110)-oriented ZnTe wafer, in which the transient electric field of THz pulse is obtained via free-space electro-optical sampling. The weak sampling beam is detected with a balanced photodetector that is connected to a lock-in amplifier. The phase of the lock-in amplifier is locked to an optical chopper that modulates either the THz generation arm or the pump beam at a typical frequency of 500 Hz. The THz beam is enclosed and purged with dry nitrogen to avoid water vapor absorption.

It should be noted that the spot size of the pump beam on the sample position should be at least two-times larger than that of the THz beam whilst conducting the OPTP experiment, which ensures a relatively uniform photoexcited region for the sample of THz probe. In addition, the thickness of the sample should be much smaller than the penetration depth 1/α, with α of absorption coefficient. In the experimental setup shown in Figure 1, what can be measured includes: (i) Adjusting the probe delay line ensures that the transmitted THz electric field is fixed around the peak value, and then the dynamical relaxation of the THz PC of the sample can be obtained by scanning the pump delay time. (ii) By fixing the delay time between the pump pulse and the THz probe pulse, the transmitted THz wavefront can be sampled by scanning the probe delay line, and the complex THz PC dispersion (Δσ = Δσ_r + iΔσ_i) is then obtained by applying a fast Fourier transform. Assuming that a thin film with a thickness d is supported on a substrate with a thickness d_s, the THz transmission (T(ω, t)) is defined as the ratio between the transmitted THz electric fields with the pump pulse at the delay time t (E_on(ω, t)) and without a pump (E₀(ω, t)). Under thin-film approximation, T(ω, t) reads [28,29]:

$$\mathrm{T}\left(\omega ,\mathrm{t}\right)=\frac{E_{on}\left(\omega ,t\right)}{E_0\left(\omega ,t\right)}=\frac{1+n_s}{1+n_s+Z_0\Delta \sigma d}=\mathrm{A}\left(\omega ,\mathrm{t}\right)e^{i\phi }$$

(1)

Obviously, A (ω, t) is the amplitude ratio of the Fourier-transformed THz signal for the excited sample at the delay time t and the unexcited sample, and φ is the phase difference between them. n_s and Z₀ are the refractive indexes of the substrate and free-space impedance, with Z₀ ≈ 377 Ω. Then, the THz PC at the delay time t is expressed by:

$$\Delta \mathsf{\sigma }\left(\omega ,\mathrm{t}\right)=\frac{1+n_s}{Z_{0}d}\left(1-\frac{1}{T\left(\omega ,t\right)}\right)=-\frac{1+n_s}{Z_{0}d}\frac{\Delta T\left(\omega ,t\right)}{T_0\left(\omega \right)}=-\frac{1+n_s}{Z_{0}d}\frac{\Delta E\left(\omega ,t\right)}{E_0\left(\omega \right)}$$

(2)

where ΔT = T − T₀ and ΔE = E_on − E₀ with T(E_on) and T₀ (E₀) of the complex THz transmission (electric field) with and without a pump, respectively. Accordingly, in the complex THz PC, $\Delta \mathsf{\sigma }=\Delta {\sigma }_r+i\Delta {\sigma }_i$ of the real (Δσ_r) and imaginary (Δσ_i) parts of THz PC is then expressed by:

$$\Delta {\sigma }_r\left(\omega ,t\right)=\left(\frac{\cos \phi }{A\left(\omega ,t\right)}-1\right)\frac{1+n_s}{Z_{0}d}$$

(3a)

$${\Delta \mathsf{\sigma }}_i\left(\omega ,t\right)=-\frac{\left(1+n_s\right)\sin \phi }{A\left(\omega ,t\right)Z_{0}d}$$

(3b)

The frequency and delay-time dependence of the real and imaginary parts of PC is manifestation of the photocarrier transportation of materials. Conventionally, THz PC dispersion can be modelled using Drude model [30,31], Drude–Smith model [32], and Drude–Lorentz model [30,31], depending on the photocarrier scattering mechanisms. The Drude model describes the free carrier response with an average momentum scattering rate, Γ:

$$\Delta \mathsf{\sigma }=\frac{D}{\mathsf{\Gamma }-i\omega }$$

(4)

where D = Ne²/m* is Drude weight, with N, e, and m* denoting the photocarrier density, electron charge, and photocarrier effective mass, respectively. Γ = 1/τ and ω are, respectively, the momentum scattering rate and angular frequency with τ of the momentum scattering time. In order to extend the Drude model to a wide range of materials with a photocarrier localization, a modified Drude model was introduced by Smith; the Drude–Smith model is expressed by [32]:

$$\Delta \mathsf{\sigma }=\frac{D\tau }{1-i\omega \tau }\left(1+\frac{c}{1-i\omega \tau }\right)$$

(5)

where the constant c varies between 0 and −1, which presents the photocarrier backscattering effect, and sometimes it refers to the localization parameter of the photocarrier. It is clear that the Drude model is recovered with c = 0, and the full carrier localization effect becomes stronger when c approaches −1.

The Drude–Lorentz model is used to describe PC dispersion--involved quasi-free photocarrier and resonance feature in the spectra, which is a combination of the Drude model and the Lorentz model, which is expressed by:

$$\Delta \mathsf{\sigma }=\frac{{\epsilon }_0{\omega }_p^2\tau }{1-i\tau \omega +i{\omega }_0^2/\omega }$$

(6)

where ω_p, τ, ε₀, and ω₀ are the plasma frequency, average momentum scattering time, vacuum permittivity, and the angular frequency of the oscillatory response, respectively. The Lorentz model is used to describe the low-frequency absorption modes, such as phonon, exciton, and plasmon.

In addition to the OPTP method, transient absorption (TA) and terahertz emission spectroscopy (TES) are also used to access the photocurrent and/or photoconductivity.

Table 1. The comparison of different time-resolved spectroscopy in 2D materials.

Time-Resolved Technique	Time Resolution	Spectra Range	Applicable Systems
OPTP	~500 fs	0.2–10 THz	Semiconducting, semimetal, and metallic systems
TA	~100 fs	Ultraviolet–Visible–Infrared	Exciton, plasmon, and optical phonon
TES	~100 fs	0.2–10 THz	Semiconducting, semimetal, and metallic systems

presents a comparison of the three methods for evaluating photoconductivity and/or photocurrent in 2D materials.

3. Regulating THz PC in 2D Materials by Gas Absorption

3.1. THz PC Regulation in Graphene by Gas Absorption

Since the discovery of the linear band dispersion and the Dirac electrons with Fermi velocity in graphene, the utilization of the Dirac electrons in 2D materials for information storage devices has been the focus of research [1]. In 2009, Zhang et al. reported that an applied voltage in bilayer graphene might open a gap in graphene [33], thus achieving the purpose of on-chip applications for the high-mobility Dirac electrons in 2D graphene. In 2012, as shown in Figure 2a, George et al. observed the negative THz PC in CVD-grown graphene following optical excitation, implying that graphene may have properties that are different from those of general semiconductors [34]. Planken’s group attributed the light-induced transparency in graphene to the generation of stimulated THz radiation and successfully achieved THz emission by combining graphene with metal nanostructures, as illustrated in Figure 2b [35].

In 2014, Ivanov et al. suggested that the increase in light/THz-induced scattering rate and the decrease in in-band conductivity should be caused by the thermalization process of intraband carriers, as shown in Figure 2c,d [36]. That means that, after photoexcitation, the intraband-transition-induced hot carriers should thermalize the carriers near the Fermi surface, and then the chemical potential of graphene is modulated, resulting in the decrease in the conductivity near the Fermi surface. Based on the discussion about the intrinsic conductivity properties in graphene, the influence of external atmosphere on the THz conductivity of graphene is one important aspect. In 2012, Docherty et al. studied the photoinduced THz conductivity in the vacuum and an oxygen and nitrogen atmosphere, and the THz PC of graphene changed from negative to positive, as shown in Figure 3 [37]. At that time, this phenomenon was attributed to the fact that the graphene monolayer is modulated by the atmosphere, which induces the opening of the band gap and makes graphene a high mobility semiconductor.

In 2017, Zhang et al. reported the physical understanding of THz PC modulation in CVD-grown graphene after photoexcitation in different atmospheres. The photoinduced THz PC cooling process was shown to be modulated by the physical adsorption of different gas molecules at the graphene surface [38]. Figure 4 demonstrates that O₂ absorption leads to decease in Fermi level of graphene, which is equivalent to the optical pump. By using the thermodynamic model, they demonstrated that the Fermi level of graphene can be modulated by adsorbing gas molecules, leading to a decrease in the chemical potential of graphene, and as a result, a decrease in THz PC can be expected [39]. Consequently, the decrease in THz PC can also be achieved by increasing the pump fluence. Microscopically, efficient electron–electron interactions convert the energy of the current into the heat energy of graphene’s entire electron gas almost instantaneously, which elevates the electronic temperature, smears out of the Fermi–Dirac distribution, and lowers the electron gas chemical potential. This further results in a reduced conductivity in the THz-frequency band [40]. Based on the reproducibility of the gas adsorption experiments, the relationship between THz conductivity and atmosphere in graphene can be mainly attributed to the physical adsorption of gas molecules on the graphene surface [41]. The characteristic of the Fermi level modulation by physical adsorption had also been applied to graphene-based gas sensors [42].

3.2. THz PC Regulation in TMDs Using Gas Absorption

The use of 2D materials in technological applications has gained increasing attention in recent decades. The tunable and layer-dependent bandgap, chemical and thermal stability, and mechanical strength make 2D TMDs viable potential components for emerging electronic devices [43,44,45,46,47]. In addition, the photoluminescence spectrum of TMDs can be adapted in various ways, including chemical doping [48] and class of substrate [49] and dielectric environment [50] choice, amongst others [51]. Because of their high surface-to-volume ratio, TMDs have excellent sensing potential. Numerous studies have focused on how the surroundings impact mono- and few-layer TMD devices [52,53]. Specifically, MoS₂ sensitivity to NO and NO₂ has been demonstrated [54]. It has also been shown that the conduction type of WSe₂ is highly sensitive to pressure [52]. Notably, studies suggest that PdSe₂ acts as an excellent gas sensor [53].

In their single-layer form, TMDs enable enhanced gate control in easy-to-fabricate transistors that are immune to short-channel effects. The ability to modulate either n-type or p-type charge transport is a key advantage of TMD transistors [55]. For example, Park et al. [56] and Qiu et al. [57] reported that adsorbed O₂ substantially reduces the current in MoS₂ field effect transistors (FETs). Grillo et al. investigated the influence of air pressure upon the carrier mobility of multilayer PtSe₂ back-gated FET: the p-type PtSe₂ FET exhibited positive PC under high vacuum in contrast to a negative PC when the pressure is raised [58]. In addition, just as the O₂ “photo-cleaning” effect observed in graphene [58,59], Zhang et al. reported that the observed, higher n-doped behavior in CVD-grown MoS₂ turned to p-dopants following laser ablation in an oxygen atmosphere [60,61,62]. Additionally, Tongay et al. demonstrated that the n-dopants in MoS₂ can be neutralized by atmospheric adsorbates [63,64]. Furthermore, the attachment of gaseous molecules has a substantial impact on the optical profiles of MoS₂. For instance, adsorbed gases can strengthen the photoluminescence of monolayer MoS₂ [65], and adsorbed O₂ modulates the dielectric function of monolayer MoS₂ [66].

The effect of O₂ on the THz PC of these 2D materials could be crucial for device realization. As shown in Figure 5, Xing et al. observed that the THz PC of a single layer of molybdenum disulfide changed with the ambient gas: negative in nitrogen and positive in air/oxygen. This provides a simple method for regulating the photoelectric properties of 2D materials [67]. The phenomenon of THz PC tuning is more likely to occur in an n-doped MoS₂. After photoexcitation, in the presence of oxygen, the mechanism of THz PC in MoS₂ monolayers changes from being dominated by trions to being dominated by free-charge carriers [68,69].

Gustafson et al. [70] also investigated the influence of molecular oxygen on the PC of MoS₂ via transient THz spectroscopy. As shown in Figure 6, when the environment changes from a vacuum to atmospheric pressure, the THz photoconductive polarity of MoS₂ transitions from negative to positive. They attributed this behavior to the physical adsorption of O₂ from the n-type MoS₂ using extra electrons [70].

4. Regulating THz PC in 2D Materials with Electrostatic Gating

4.1. Regulating THz PC of Graphene with Electrostatic Gating

Charge carrier in graphene exhibits high mobility and strong interaction with electromagnetic radiation over a wide range of frequencies [71,72], which provides a good platform for testing electronic and optical response of 2D materials following optical excitation. In particular, OPTP spectroscopy provides access to a transient THz PC response. It was noted that positive THz PC was demonstrated in epitaxial graphene on a SiC substrate [73], while negative THz PC was observed in graphene-grown fused silica and quartz substrates by chemical vapor deposition (CVD) [74,75,76]. It has been thought that the negative THz PC in CVD-grown graphene may arise from THz-stimulated emission with optical excitation [77]. Later, it was noted that the vast difference between epitaxial-growth graphene shows very-low intrinsic carrier density, while the CVD-grown graphene conventionally shows p-doped with a much lower Fermi energy level. As a result, people argued that the opposite THz PC observed in graphene fabricated with different technique arises from their different charge densities [78,79,80]. Therefore, regulating THz PC is applicable by adjusting the intrinsic carrier density, i.e., the Fermi level of graphene, which has been studied extensively in recent decades. At present, it is clear that graphene shows positive THz PC when the Fermi level is close to the Dirac point, i.e., neutral charge graphene, in which carrier interband scattering plays a dominating role following optical excitation. On the other hand, graphene shows negative THz PC when the Fermi level is distant from the Dirac point that corresponds to doped graphene; in this case, the carrier intraband scattering accounts for the observed negative PC. In order to observe the transition from the positive PC to a negative one in graphene, a popular method uses OPTP spectroscopy combined with electrostatic gating.

In 2014, Frenzel et al. [78] reported the semiconducting-to-metallic PC crossover in graphene by using the electrostatic gating method. In the same year, Shi et al. [79] conducted a similar study. Figure 7 schematically illustrates the key concept that the authors attempted to study. By adjusting the gating voltage V_g, the doping carrier density can be controlled; as a result, the negative-to-positive THz PC is controllable, which can be clearly seen with OPTP spectroscopy.

The biased, dependent THz PC of the graphene device was measured with a conventional OPTP spectroscopy at room temperature in high vacuum. The sample was photoexcited with a 100 fs laser pulse at a central wavelength of 800 nm. The typical experimental results are shown in Figure 8, in which the transmitted THz pulse is collected at the condition of the pump fluence of 10 μJ/cm² with biased voltages of V_g − V_CN = 2 V and V_g − V_CN = 52 V. It is obvious from Figure 8a that the change in the transmitted THz field (ΔE) shows an opposite sign to that of the incident THz pulse for the case of V_g − V_CN = 2 V, reflecting a photo-enhanced THz absorption. The extracted THz PC dispersion, Δσ = Δσ_R(ω) + iΔσ_Im(ω), shows a positive real part (Δσ_R(ω) > 0) (Figure 8b). In direct contrast, ΔE shows the same sign as that of the incident THz pulse when V_g − V_CN = 52 V, as shown in Figure 8c, which reveals the photoinduced decrease in THz absorption; correspondingly, the extracted real part of THz PC is negative, as expected (Δσ_R(ω) < 0) (Figure 8d). Figure 8e shows the transient THz electric field transmission with various biases, which clearly demonstrates that the transmitted THz field changes with the applied bias. By considering the interplay between photoinduced carrier population change (Δn) and scattering rate (Γ), the authors modelled the THz PC dispersion, and the modelling results showed agreement with the experimental data qualitatively [78,79].

Regarding the mechanism that governs the sign of THz PC in graphene, Tomadin et al. [80] performed a similar experimental study, and they reported similar experimental phenomena: the sign of THz PC transforms from negative at a near neutral charge doping to a positive one at a high charge doping. They presented an alternative explanation for the experimental data: The electronic screening plays a crucial role in the substrate-supported graphene thin film. In the case of charge neutrality, there is a greater free-carrier density available for conductance in the event of interband heating, and this pre-empts the effect of the reduced screening at a higher electron temperature. The THz PC is positive. For the highly doped graphene, where intraband scattering heating occurs, the system evolves to a state that weakens the shielding effect of substrate impurities, and the THz conductivity decreases with respect to the equilibrium value. The THz PC is negative. According to this non-regularized dynamical screening model, both carrier multiplication and interband recombination due to carrier–carrier scattering disappear [80].

Although electrostatic gating on graphene has been studied extensively, there is the challenge of regulating PC in few-layer TMDs via electric gating. For the semiconducting TMDs, such as MoS(Se)₂ and WS(Se)₂, the insulating nature of TMDs makes tuning carrier density in TMD difficult with a conventional voltage of a few volts to tens of volts. For (semi)metallic TMDs, such as WTe₂- and 1T′-phase MoTe₂, the electrical regulation of the TMDs could be limited by the challenges from the stacking and electrode growth technique of the heterostructures.

4.2. THz PC of Gr/TMD Heterostructures Regulated Using a Substrate Field

One of the most critical components of electronic or optoelectronic devices is the metal-semiconductor junction (MSJ) structure [81]. The Schottky barrier height (SBH, Φ), the energy barrier height for charge carrier transport across the junction, is a significant parameter for a MSJ that impacts device performance [82]. Efforts to tune Φ in 2D TMD-based devices have recently been increased because of their unique properties and photoelectric engineering applications [83,84,85]. However, experiments show that metals with different work functions have little effect on the Φ of metal–2D semiconductor heterojunctions, and the Fermi level of the heterojunction is essentially near the semiconductor band gap, with little change relative to the different metals used, and changing the type of metal has limited ability to regulate the Fermi level of the MSJ. Therefore, it is difficult to regulate Φ using different common metals because of the effect of Fermi level pinning (FLP) [86].

A recent study by Liu and colleagues [85] demonstrated that 2D metals present a viable solution to the challenging FLP problem. According to their findings, FLP is not effective for MSJs containing vdW interactions owing to the inhibition of metal-induced gap states in the semiconductor. As a result, the adjustment of the energy level alignment can be achieved using diverse 2D metals [86]. As shown in Figure 9, when the Fermi level is near to the lowest-lying states of the semiconductor, there is an energy difference ${\mathsf{\Phi }}_{B,n}^0$ above the Fermi level, known as the n-type Schottky barrier contact, i.e., it is easier for electrons to flow from the semiconductor to the metal compared with flowing in the opposite direction. When the Fermi level is near to the highest-lying states of the semiconductor, there is an energy difference between the Fermi level and the valance band maximum (VBM), marked ${\mathsf{\Phi }}_{B,p}^0$, i.e., p-type Schottky barrier contact, which controls the transport of holes across the MSJ interface [82].

Owing to its exceptional strength and excellent electrical and thermal conductivity, graphene has been widely used in various applications, including sensors, integrated circuits, and FETs [87,88,89,90]. In recent years, researchers have investigated numerous 2D heterostructures involving graphene and different materials, including but not limited to Gr/TMD heterostructures. Owing to its low electrical resistivity and high electron transfer rate, graphene effectively preserves its distinct attributes when combined with the intrinsic electronic characteristics of other 2D materials, such as Gr/TMDs heterostructures [91,92]. In Gr/TMD heterostructures, the SBH can be modified by applying a vertical electric field to the heterostructure. By applying an increasing vertical electric field to the Gr/WS₂ heterostructure, Zheng et al. observed a transition from an n-type to a p-type Schottky contact, indicating a significant change in the electronic properties of the material [93]. By adjusting the electric field, Yu et al. found that the Schottky barrier height of Gr/WSe₂ van der Waals heterostructures can be altered. This demonstrates that the electronic properties of the material can be manipulated using external forces [94]. A perpendicular electric field can modulate the electronic properties of the Gr/WSe₂ vdW heterostructure, as stated by Sun et al. [95].

For electronic and optoelectronic applications, interlayer coupling and charge transfer through the interface of Gr/TMD heterostructures play a critical role in modulating their electronic properties. However, it is not clear whether the charge transfer is electron or hole. Zhang et al. found that when light is absorbed by the MoS₂ layer in Gr/MoS₂ heterostructures, it generates electron–hole pairs that are subsequently separated across the layers. Specifically, the photoexcited electrons migrate from MoS₂ to graphene, while the photoexcited holes become trapped in the MoS₂ layer [96]. By using time-resolved OPTP spectroscopy, Xu et al. demonstrated that a Gr/WSe₂ heterostructure enabled the transfer of holes from WSe₂ to graphene, effectively lowering the Fermi level of graphene [97]. Zhu et al. verified that it is feasible to extract hot electrons from graphene post-carrier intraband scattering, but prior to the electron–hole interband thermalization. Their measurements showed a rapid electron injection (~25 fs) from graphene to WSe₂ when subjected to sub-bandgap excitation [98].

The influence of substrates on 2D materials is a result of charge transfer, doping monolayers, or dielectric screening [99]. The dielectric screening effect through substrates can impact both layers in a heterostructure as the charge screening length is greater than the thickness of the atomic layer [100]. In Gr/TMD heterostructures, the interfacial charge transfer can be affected by the effective electric field induced by the substrate [101,102,103]. For instance, in a Gr/WSe₂ heterostructure, the Dirac cone is commonly located adjacent to the valence band maximum of the WSe₂ layer. This configuration suggests the presence of a standard p-type Schottky barrier contact at the interface [95]. Xing et al. used OPTP spectroscopy to measure the ultrafast carrier dynamics in both WSe₂/Gr/sapphire (WSe₂/Gr) and Gr/WSe₂/sapphire (Gr/WSe₂) heterostructures. They performed this measurement by photoexciting the samples with photons of various energies spanning a range of 0.7–2.5 eV. The efficient electric field of the sapphire substrate induces a reduction in the electron barrier height in Gr/WSe₂, thereby transforming the Schottky barrier contact into an n-type. This alteration makes the transfer of photo-thermionic electrons from graphene to WSe₂ over the Schottky barrier more advantageous. Therefore, for photon energy values below the WSe₂ A-exciton excitation, i.e., only the graphene layer is excited, the relaxation time in Gr/WSe₂ is longer than that of graphene. However, in WSe₂/Gr, the barrier height for electrons increases, leading to a blocked photo-thermionic electron transfer. As a result, the lifetime of WSe₂/Gr is approximately similar to that of graphene (see Figure 10) [101].

Ma et al. explored the process of charge transfer in the heterojunction of five-layer PtSe₂/Gr, which was placed on a fused silica substrate. The findings presented in Figure 11 demonstrate that the direction of the electric field, which is consistently perpendicular to the substrate, can either facilitate or hinder the transfer of charges. Under a low pump fluence, the effective electric field introduced by the substrate promotes the transfer of photoexcited electrons in the PtSe₂/Gr/substrate configuration, resulting in negative THz PC. As the pump fluence increases, excess photo-generated carriers begin to screen the electric field introduced by the substrate, gradually causing the negative THz PC to transition into a positive one. In contrast, in the Gr/PtSe₂/substrate, no significant charge transfer can be detected, as indicated by the fact that the THz PC remains positive, regardless of the pump fluences applied and excitation wavelengths used [102]. The modulation of the electric field induced by the substrate can alter the height of the Schottky barrier, thereby allowing the regulation of the THz PC.

5. Regulating THz PC in Gr/TMD Heterostructures by Photogating

In addition to gas adsorption and electric field by a substrate, a more direct way to regulate the PC of graphene is to use graphene-based 2D vdW heterojunctions. There are several methods to control the THz PC in such heterojunctions. In this paper, we first discuss the use of light itself to generate and control the positive and negative THz PC and, thus, verify the charge transfer process in 2D materials. In 2021, Fu et al. studied the charge transfer process in a Gr/WS₂ heterostructure with the OPTP technique for below and above band gap excitation [104]. As shown in Figure 12, they found that, when the heterojunction is excited below the bandgap of WS₂, the electron transfer occurs from graphene to WS₂. A direct hole transfer takes place from WS₂ to graphene by exciting heterojunctions above the WS₂ bandgap. The observed positive THz PC arises from the photogating effect on graphene. Considering that the carrier mobility of graphene is at least two-orders of magnitude higher than that of WS₂, the photoinduced charge transfer between graphene and WS₂ results in an increase in the hole concentration in graphene for both below and above band gap excitation. As a result, the THz PC is positive.

As shown in Figure 13a, Ma et al. [102] reported the changes in positive and negative THz PC in a PtSe₂/Gr heterojunction and found that the changes in positive and negative THz PC could be modulated by different stacking orders, which may be related to the formation of effective point fields by carriers’ accumulation at the interface [102]. Later, in 2022, Quan et al. investigated this charge transfer method caused by carrier aggregation. It was found that it could be related to the charge transfer process caused by the thermal phonon bottleneck effect in graphene, with the analysis of the scattering time and the Smith parameter in the heterostructure, as shown in Figure 2b [105].

In 2022, Zou et al. reported that the process from negative to positive THz PC in the heterojunction between graphene and MoS₂ is related to the formation of interlayer excitons between graphene and MoS₂, and the formation time of such interlayer excitons was determined for the first time to be around 18 ps [106]. Furthermore, they also found the dependence of the laser fluence between the hot electron transport and photogating process by theoretical calculations. Figure 14 shows the dynamical formation of interlayer exciton in Gr/MoS₂ heterostructure under below and above bandgap excitation, investigated with transient spectroscopy and time resolved THz spectroscopy.

To summarize, the photogating effect has been widely used to achieve the regulation of THz PC in graphene-based heterostructure, in which optical fluence, wavelength, as well as the heterostructure stacking order are the adjustable parameters to control the THz PC, including the sign and magnitude of the PC. It should be noted that the photogating effect could be applicable for mostly similar van der Waals structures. In addition, the photogating effect is also used to study the charge transfer process, such as the direction of charge transfer, the type of transferred carriers, and the amount of transferred carriers in graphene-based heterostructures [107]. This control can be considered as the photogating process of the Schottky barrier of the structure and has also been applied to graphene-based photodetectors.

6. Regulating THz PC in TMDs by the Photoinduced Formation of Elementary Excitation

THz PC can be used to probe the photoinduced formation of low-energy elementary excitations. The system lies at far out of equilibrium upon photoexcitation, so that the transport properties are very different from the equilibrium condition. The formation of low-energy elementary excitation following photoexcitation leads to a significant change in THz PC. Therefore, OPTP spectroscopy can be used to detect the newly formed quasi-particle, on the one hand, and the formation of the new quasi-particle results in the carrier mobility change that can be used to regulate the THz PC, on the other hand. In this paper, we briefly review the photoinduced formation of a trion (a charged exciton) and small and large polarons by using time-resolved THz spectroscopy.

6.1. Regulating THz Photoconductivity via the Formation of Trion in Monolayer MoS₂

Atomically thin transition dichalcogenides (TMDs) is a 2D semiconductor, which has been demonstrated to feature a strong quantum confinement and reduced dielectric screening effect [108,109,110,111]. The resulting strong Coulomb interaction causes the photoexcited electron–hole pairs to form tightly bound excitons, which determine the electronic and optical properties of the materials [109,111]. For most monolayer TMDs, unintentional doping is inevitable, which leads to TMDs with excess charges with n- or p-doping. Excitons formed by photoexcitation can capture additional charges to form trions, or charged excitons [112,113]. These trions possess very different mobility as that of the excess charge carrier. As a result, the formation of trions can significantly affect the THz PC of the system [114]. Therefore, time-resolved THz spectroscopy is a sensitive tool to probe the formation of trions; vice versa, the formation of trions can also be used to regulate the THz PC.

Trion-induced negative THz PC was first studied in a monolayer MoS₂ by Lui et al. in 2014 [114]. The authors used a conventional optical pump and THz probe spectroscopy, in which a Ti: sapphire amplifier system generated a 90 fs laser pulse with a repetition rate of 5 kHz and a photon energy of 1.55 eV. The laser was split into two beams. One beam was guided into an optical parametric amplifier to output a frequency tunable pulse, which acts as a pumping beam. The other beam was used to generate THz radiation in the ZnTe crystal for probing the samples. The experiment was conducted in a high vacuum with a variable temperature. In this study, the monolayer MoS₂ on sapphire substrate was fabricated with the CVD method; the electron concentration and the carrier mobility in the monolayer MoS₂ were estimated to be ~5.5 ± 2 × 10¹² cm⁻² and 180 ± 70 cm²V⁻¹s⁻¹, respectively. The typical experimental results are shown in Figure 15, and Figure 15a presents the time-domain THz transmission of MoS₂/sapphire as well as a reference (bare sapphire). By referring to the sapphire substrate, the complex sheet conductivity of the monolayer MoS₂ can be obtained, from which the real part of average conductivity is σ₁ ≈ 2 ± 0.5 G₀ with G₀ = 2e²/h of the quantum of conductance. Following 400 nm optical excitation, the pump-induced change in THz transmission (proportional to ΔE) is shown in Figure 15b, and it is clear that the wavefront of ΔE shows the same sign as that of the incident THz pulse, which indicates that THz transmission is enhanced with pump excitation. Figure 15c further demonstrates this transient-pump-induced increase in THz transmission. It is clear that dynamical THz transmission exhibits a short component with a lifetime of 1 ps, followed by a long component with a lifetime of 42 ps. Figure 15d presents the extracted photoconductivity dispersion at a delay times of 3 ps (top panel) and 40 ps (down panel). It should be noted that both the real and imaginary parts of photoconductivity are negative for all delay times and in the investigated frequency.

The pump-induced negative THz PC in the monolayer MoS₂ is accounted for the formation of trions, which is depicted schematically in Figure 16 [114]. The monolayer MoS₂ film on the sapphire substrate fabricated with the CVD method behaves as an n-type semiconductor, which slightly attenuates the THz wave as illustrated in Figure 16a. With photoexcitation, the resulting electron–hole pairs form trions with the excess free charges on a very fast time scale. Trions behave as free charged particles with an increased effective mass, resulting in a reduced carrier mobility and reduced conductivity. As the photoexcitation does not increase the pure carrier concentration, instead, photoexcitation adds mass to the original free charges, leading to a decrease in conductivity and an increase in THz transmission, as illustrated in Figure 16c.

6.2. Regulating THz Photoconductivity by the Formation of Polarons

Similar to the formation of a trion via photoexcitation in a semiconductor, P. Suo et al. [115] reported the observation of negative THz photoconductivity in a Dirac semimetal PtTe₂ thin film with an optical pump and THz probe spectroscopy. The typical experimental findings are displayed in Figure 17, in which the PtTe₂ films with various thickness were fabricated by tellurizing Pt thin films on a fused silica substrate. Figure 17a shows the transient THz transmission, ΔT/T₀ at 780 nm with a pump fluence of 542 μJ/cm². The inset presents the zoomed-in view of a short time scale. It is obvious the transient transmission of THz frequency contains three phases with a different time scale: (1) a pump-induced rapid drop in THz amplitude transmission; (2) the rapid transition from positive to negative THz PC signal on sub-picosecond time scales; and (3) the slow recovery from the maximum bleaching signal to the equilibrium state. Figure 17b presents the dynamical evolution of THz transmission with various pump fluences. Considering that the bleaching signal lasts for a few ns, an exponential function convoluted with a laser pulse was used to fit the transient THz transmission of Figure 17a in the initial 10 ps time window, and Figure 17c displays the fitting time constant with respect to the pump fluence. Figure 17d shows the transmitted THz waveforms through the film at the delay times of 0 ps (red) and 5 ps (blue) as well as without photoexcitation (black). By comparing with the transmitted THz waveform without pumping, the photoinduced change in the THz electric field, ΔE, shows to be out of phase at the zero delay time and in phase at the delay time of 5 ps. The in-phase characteristic for blue and black indicates the photoinduced bleaching in THz transmission, or rather, photoexcitation leads to the increase in THz transmission.

Before assigning the negative THz PC to the excitation of a small polaron in the PtTe₂ film, authors discussed and ruled out other possible contributions, such as interband scattering, i.e., by exciting a Dirac fermion into a trivial conduction band, formation of trions [114], hot electrons effect [116], impurity effect [117], and THz emission [118]. Then, authors concluded that the photoinduced negative THz PC arises from the photoexcitation-induced formation of a small polaron. Notably, early experimental measurements [119] show that PtTe₂ exhibits a strong e–ph interaction, with a coupling constant ranging from 0.38 to 0.42. Photogenerated electrons can be strongly coupled to the lattice vibration of PtTe₂ under photoexcitation. It is this strong e–ph coupling that makes possible the cooling of hot electrons and the deformation of the lattice around the carriers, resulting in small polarons with a reduced mobility. In order to consolidate the photoinduced formation of a small polaron, the authors also performed OPTP studies with varying values of pump fluence, temperature, and PtTe₂ film thickness, and all the experimental data supported the formation of a small polaron. The authors also evaluated carrier mobility and the pump-induced mobility change in the 20 nm PtTe₂ film in THz frequency. The average carrier mobility μ₀ was calculated to be 1.5 cm² V⁻¹ s⁻¹ based on the THz conductivity and carrier concentration data. The pump-induced carrier mobility change was then obtained to be Δμ/μ₀ = (μ − μ₀)/μ₀ = 10.5% at a delay time of 3 ps under the absorbed photon density of 3.2 × 10¹⁴ cm⁻². The significantly reduced carrier mobility at a high pumping fluence was indeed consistent with the model of small polaron formation and is due to lattice deformation caused by photogenerated thermal electrons in PtTe₂ films.

The photoinduced formation of a small polaron has also been reported by Chen et al. [120] in the low temperature phase of T_d MoTe₂ films. The typical results are displayed in Figure 18. After the photoexcitation of 780 nm, the 1T′-phase MoTe₂ at a high temperature exhibits only positive THz PC and its relaxation time is less than 1 ps. When the temperature is lower than 200 K, MoTe₂ film undergoes phase transition, from a high temperature 1T′-phase to low temperature T_d-phase MoTe₂ [121,122]. While the low-temperature T_d-phase MoTe₂ film showed an ultrafast positive THz PC, this was initially followed by a negative THz PC, and the negative THz PC signal lowers to the equilibrium state in the hundreds-of-ps time scale. The authors proposed that small polaron formation induced by hot carrier is ascribed to the negative THz PC in the polar semimetal MoTe₂ at a low temperature [120]. The polaron formation time increased slightly with the temperature and was determined to be ~0.4 ps at 5 K and 0.5 ps at 100 K.

In addition to probing small polaron formation, THz spectroscopy has also been used to investigate the dynamic formation and band transport of large polarons in 2D materials [123,124,125,126]. Employing time-resolved THz spectroscopy, Bretschneider et al. [124] studied the temperature- and pump-energy-dependent rising process of THz PC in three organic–inorganic hybrid metal halide perovskites, CH₃NH₃PbI₃, CH(NH₂)₂PbI₃, and CsPbI₃. They found that the rise in PC can be well described by a simple model, in which the initial carrier cooling and large polaron formation occur in succession [124]. The polaron formation time was determined to be ~400 fs for the three investigated perovskite films. Jin et al. [125] demonstrated the use of the OPTP technique to access the nature of the interplay of photoexcited unbound charge carriers and optical phonons in polycrystalline CH₃NH₃PbI₃ with a grain size of about 10 μm. By using the Drude–Smith–Lorentz model and Fröhlich-type e–ph coupling, they came to the conclusion of the formation of large polarons of a much longer charge lifetime in polycrystalline perovskite films, which is a result of the efficient protection of band-edge carriers by the trap potential and low thermal conductivity. Zheng et al. [126] employed the OPTP technique to investigate the temperature-dependent THz PC of two model MXenes: semiconducting Nb₄C₃T_x and metallic Ti₃C₂T_x, in which Nb₄C₃T_x nanoflake behaves as a positive THz PC response, while negative THz PC occurs in Ti₃C₂T_x nanoflake following optical excitation. The experimental results obtained from transient THz PC revealed that the band-like transport dominates the short-range, intra-flake charge transport in MXenes. This is in contrast to previously static electrical transport measurements [127,128], in which inter-flake transport occurs by hopping and becomes the limiting step for charge percolation through the network of MXene flakes. Furthermore, they also reported that carrier-LO phonon scattering plays a dominating role in intra-flake carrier transport for both semiconducting and metallic MXenes [126]. This study reconciled the debate between theoretical and static electrical transport studies on the charge transport mechanism, and a unifying picture was proposed to depict the charge transport in MXenes [126].

As one of most investigated 2D materials, graphene exhibits linear band dispersion, high carrier mobility, and strong light–matter interaction over a broad band frequency. Plasmon in a structured graphene is widely used to regulate the optical response and THz photoconductivity. Graphene plasmonics shows some advantages as compared to metal plasmonics. The plasmonic Drude weight is dominated by the graphene carrier density, which shows a high tunability with electrical field, chemical doping, or optical excitation. The low electronic density of states and relatively weak electron–phonon coupling makes graphene to have very-high carrier mobilities, attainable via graphene enclosed within hBN multilayers. In addition, electromagnetic fields can be confined into a dimension that is much smaller than the wavelength of a photon, which promotes the formation of plasmon–polariton interaction, an elementary excitation originating from strong interactions between photons and plasmon. The tight electromagnetic field confinement and large density of photonic states allows plasmon–polariton to achieve novel and otherwise inaccessible functionalities. Recently, polaritons, including plasmon–polariton interactions, have received great interest in the broad fields of physics, materials science, and photonics for both fundamental research and potential applications [129,130,131].

7. Conclusions and Perspective

Tailorability, particularity, and easy fabrication are behind the extensive attention that research on 2D vdW materials has received in the past two decades. Although graphene has a weak light absorption, most other 2D materials show a strong absorption in the visible and near infrared regions, and light–graphene interaction can be greatly enhanced by stacking graphene and other 2D materials together. In addition, as the THz waves have a low photon energy, low loss to 2D materials, and high sensitivity to free carriers near the Fermi surface, the THz spectrum becomes an effective means to characterize the charge transfer at the surface of materials in many fields, such as condensed matter physics, materials science, biology, and chemistry. Applying THz PC spectroscopy to 2D materials can reveal the fundamental properties of 2D materials, such as identifying elementary excitation, distinguishing charge transport mechanisms, and scattering mechanism. Regulating THz PC can be realized by designing 2D materials with the desired functionality or stacking different 2D materials together. As a result, THz optoelectronic devices, such as THz modulator, switch, and transistor, can be designed and fabricated. In this review, we comprehensively reviewed the latest progress on the regulation of THz PC in 2D materials from four perspective: (i) the influence of gas absorption on the THz PC in graphene and TMD films; (ii) the effect of electric bias on the THz PC in graphene as well as the influence of substrate-introduced field on the THz PC in the Gr/TMD heterostructure; (iii) impact of photogating on the regulation of THz PC based on the Gr/TMD heterostructure; and (iv) the regulation of THz PC via the photoinduced formation of elementary excitations, such as trion, small polaron, and large polaron.

In this paper, we presented a review focused on the regulation of THz PC based on graphene, TMDs, and Gr/TMD heterostructures. As mentioned in the introduction, the members of 2D materials’ family covers from graphene to TMDs, to hBN [10], silicone [11], MXene [12], phosphorene [13], borophene [14], and heterostructures by stacking two or more different 2D materials together [3,7,15,16]. Different 2D materials process diverse structures, properties, and functionalities. Graphene and TMDs are the most-studied candidates in the 2D materials’ family. In general, the regulation of THz PC discussed in this paper is applicable to other 2D materials; of course, the detailed THz PC may be different due to the different molecular structure and inter- and/or intralayer interaction. For instance, Zheng et al. revealed that THz PC in Mexene film is dominated by intra-MXene flake transport of charge carrier; this contrasts with the static electric transport measurement, in which inter-flake charge carrier hoppling dominates the charge transport [126].

In addition to the effects of the gas absorption, electrostatic gating, photogating as well as the formation of elementary excitation on the THz PC, other methods, such as strain, magnetic field, and quantum confinement, can also play important roles in regulating THz PC. Xu et al. predicted with the first principle calculation that 1T′-phase monolayer Janus transition metal dichalcogenides (JTMDs) possess colossal nonlinear photoconductivity. They also revealed that in-plane strain can induce topological phase transitions in the 1T′ JTMDs, and the shift current can abruptly flip their directions [132]. Arora A. et al. demonstrated theoretically that large injection currents can develop in strained, twisted bilayer graphene heterostructures with a broken sublattice symmetry [133]. Akimov et al. presented an overview of optical and photocurrent spectroscopy with picosecond strain pulses [134], in which a picosecond strain pulse can be used to gate, on a picosecond timescale, the photocurrent in a p–i–n diode containing a quantum structure (including quantum well and quantum wires and quantum dots) in its intrinsic region. In addition, magnetic field can also be used to regulate photoconductivity and photocurrent. S. Candussio et al. reported the observation of edge electric currents excited in bilayer graphene by THz laser radiation [135]. The application of a small magnetic field normal to the graphene plane leads to a phase shift in the polarization dependence. The authors also revealed that the photocurrent exhibits sign alternating magneto-oscillations showing periodic in 1/B [135]. In 2D materials, photocarriers are confined in a two-dimensional space, and quantum confinement also plays an important role in regulating the photoconductivity and photocurrent in 2D materials. Silvia G. Motti et al. demonstrated that 2D perovskite (BA)₂PbI₄ exhibits surprisingly high in-plane mobilities and enhanced charge–phonon coupling with respect to the 3D counterpart (BEA)2PbI4 [136]. Last but not least, the twist angle of homo-/hetero-bilayer graphene and/or TMD structure can also have a roles in affecting the photocurrent and photoconductivity. Conventionally, introducing Moiré lattice in 2D materials twist angle can localize the carrier transport, which results in decreasing in PC. By constructing the Gr/MoS₂ bilayer with different twist angles, Luo et al. found that the charge transfer timescale from MoS₂ to graphene varies strongly with twist angle, becoming faster for smaller twist angles, and shows that the relaxation timescale is significantly shorter in a heterostructure as compared to a monolayer [137].

Although the non-equilibrium THz PC of 2D materials and vdW heterostructures have been employed to detect the carrier and energy transfer, thermalization kinetic process of the carriers around Fermi surface. However, open questions remain in these series of 2D materials, and heterostructures require extensive in-depth investigation, such as the heat transfer process [138], phonon recycling [139], transient shift current [140,141], and hot electron extraction [135]. In addition, the development of a new time resolved technique, such as the near-field THz imaging, will provide new insights into the understanding of the non-equilibrium dynamics at the angstrom scale 2D material interface. In addition to THz PC spectroscopy, THz emission spectroscopy based on the photoinduced charge current has also been demonstrated to be a powerful tool to study the charge dynamics and charge transfer. For instance, photoexcitation-induced charge transfer in a heterostructure is expected to accompany THz emission arising from the photocurrent [142]. Although a few pieces in the literature reported the observation of THz emission in the TMDs/TMDs heterostructure, there is no report about THz emission in Gr/TMD heterostructures to date, and the underlined mechanism is still not clear at present. Moreover, by fabricating a monolayer TMDs on a ferromagnetic thin film to construct a ferromagnetic heterostructure, the photoexcitation of ferromagnetic layer may lead to spin injection in the TMD layer; as a result, enhanced THz emission could be possible via inverse spin Hall effect [143,144,145,146]. In conclusion, transient THz PC spectroscopy assisted with THz emission spectroscopy can provide an insight into the photocarrier dynamics of the desired 2D materials as well as the heterostructures; the regulation of THz PC shows the uttermost important in the fundamental research of 2D materials and the heterostructure, which is also significantly important in designing and developing novel THz optoelectronic devices.