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We report spatially and temporally resolved measurements of photocarrier transfer process in van der Waals heterostructures. Graphene-WS2 hetero-bilayers were fabricated by manually stacking monolayers of graphene and WS2 obtained by mechanical exfoliation. Photocarriers were excited in WS2 by an ultrafast laser pulse. Their transfer to graphene was monitored by measuring the differential reflection signal as a function of both time and space. Surprisingly, we found that the width of the photocarrier profile in graphene decreases with time. This counter-intuitive phenomenon suggests that the Coulomb field of the holes transferred from WS2 to graphene can effectively drag the electrons to speed up their transfer. This effect illustrates that an externally applied electric field can be used to control the ambipolar photocarrier transfer in van der Waals heterostructures.
© 2017 Optical Society of America
1. Introduction
The successful isolation and identification of graphene in 2004 [1, 2] has stimulated extensive studies on other two-dimensional (2D) materials including transition metal dichalcogenides (TMDs), transition metal oxides, hexagonal boron nitride, and elementary monolayers such as phosphorene, silicine, and germanene [3–8]. In addition to their promising potential applications as individual materials owing to their novel properties, these 2D materials can be used to fabricate multilayer van der Waals heterostructures [9–11], which bring studies of functional heterostructures to a new regime with atomically sharp interfaces, exceptional mechanical flexibility, and chemical stability. The van der Waals nature of the interlayer coupling imposes little restrictions on the selection of partnering materials, removing a strong constrain in making heterojunctions with traditional materials.
Among the van der Waals heterostructures that have been investigated so far, hetero-bilayers formed by a graphene and a TMD monolayer serve as a perfect example of combining the strength of the partnering materials. Graphene possesses exceptional high charge mobilities [12], while TMDs are excellent absorbers of visible light [13]. Hence, such a structure can be used for photodetection and photovoltaic devices, where the TMD and graphene serve as light-absorbing and charge transport components, respectively. Indeed, various applications of heterostructures formed by graphene and TMD have been demonstrated. For example, photodetectors of graphene-MoS2 [14, 15] and graphene-WS2 [16] have shown high internal quantum efficiencies. Photovoltaic devices based on graphene-TMD [17] have been demonstrated, such as graphene-MoS2 and graphene-WS2 [18]. For electronic applications, transistors have been fabricated using such structures, including graphene-MoS2 [19–21], graphene-WS2 [22], and graphene-WSe2 [23]. Memory devices based on graphene-MoS2 heterostructures have also been demonstrated [24,25].
Since the Dirac point of graphene is located between the conduction band minimum and valance band maximum of most TMD monolayers [26, 27], in graphene-TMD heterostructures photocarriers excited in TMD are expected to transfer to graphene. This is a key process for effective applications of such heterostructures in optoelectronic devices. Previously, some of us have shown that in graphene-WS2 heterostructures, photocarriers excited in WS2 can transfer to graphene in 1 ps with high efficiency [28]. In heterostructures formed by different types of TMD monolayers, interlayer charge transfer was also found to be highly efficient, as evident by pronounced photoluminescence quenching effect [29–36], and ultrafast, as revealed by ultrafast laser measurements [37–41]. However, the mechanism of such ultrafast transfer processes is still yet to be understood, given the weak van der Waal nature of the interlayer coupling. Several models have been proposed on this process, such as resonant electron transfer to high energy states [42], quantum coherent effect [43], and effect of Coulomb potential [44]. Despite of the progress, experimental evidences revealing the microscopic mechanisms of charge transfer across van der Waals interfaces would be of great value.
Here we show experimental results that reveal the effect of Coulomb field on charge transfer in graphene-WS2 heterostructures. A tightly focused ultrafast laser pulse was used to inject photocarriers in the WS2 layer with a highly nonuniform spatial distribution. Due to the larger driving force from the band offset, holes are expected to transfer to graphene faster than the electrons. Once the holes are transferred, a space charge field is established with a Gaussian space profile. The stronger field at the central area of the profile causes a faster transfer of electrons in that region compared to the low-field regions, resulted in a narrowing of the spatial distribution of the photocarriers in graphene. The result revealed that the Coulomb field between electrons and holes can facilitate the interlayer charge transfer, which suggests that although the electron-hole pairs are neutral, an externally applied electric field may still be used to effectively control the carrier transfer process.
2. Experimental
We studied a graphene-WS2 heterostructure, as schemetically illustrated in Fig. 1(a). According to first-principle calculations, the Dirac point of graphene is 4.57 eV below the vacuum level [27], while the ionization potential and electron affinity of WS2 monolayer are −5.88 and −3.75 eV, respectively. Hence, the Dirac point of graphene is 0.82 eV below the conduction band minimum and 1.31 eV above the valence band maximum of WS2, as shown in Fig. 1(b). As a consequence, electrons and holes excited in WS2 are expected to transfer to graphene.
Fig. 1 (a) Schematic of the heterostructure sample formed by graphene and monolayer WS2. (b) Band alignment of graphene and WS2 monolayers. (c) Raman spectrum of the graphene monolayer used to fabricate the heterostructure. (d) Photoluminescence spectrum of the WS2 monolayer used to fabricate the heterostructure.
The heterostructure samples studied were fabricated by stacking together mechanically exfoliated monolayers. Graphene and WS2 flakes were mechanically exfoliated from bulk crystals onto polydimethylsiloxane (PDMS) substrates using adhesive tapes. Monolayer flakes were first identified by optical contrasts with a microscope. The monolayer thickness of the graphene flake was then confirmed by its Raman spectrum. As shown in Fig. 1(c), the ratio of the 2D and G peaks is consistent with previously established values of monolayer graphene [45]. The monolayer thickness of the WS2 flake was confirmed by its photoluminescence spectrum with high yield, as shown in Fig. 1(d). Next, the WS2 flake was transferred onto a silicon substrate with a 90-nm oxide cap layer. The sample was annealed under a H2-Ar (20 – 100 sccm) environment at a base pressure of about 5 Torr at 200°C for 2 hours. The graphene flake was then transferred on top of the WS2 flake. The sample was annealed again under the same condition.
In the transient reflection measurements, a Ti:sapphire laser generates 100-fs pulses with a central wavelength of 790 nm at a repetition rate of about 80 MHz. This laser beam is focused to a beta barium borate (BBO) crystal to generate its second harmonic at 395 nm. A dichroic beamsplitter is used after the BBO crystal to separate the 395-nm beam from the residual 790-nm beam. The other portion of the 790-nm beam is used to pump an optical parametric oscillator, which generates an output beam at about 1250 nm. Another BBO crystal is used to generate second harmonic of this beam at 625 nm.
The measurements were performed with 395-nm pump and 625-nm probe pulses. The two beams are combined by a beamsplitter and focused to the sample by a microscope objective lens. The reflected probe is collected by the same objective lens and is sent to a biased silicon photodiode. A lock-in amplifier is used to measure the voltage output of the photodiode. A mechanical chopper is place in the pump arm in order to modulate its intensity at 2 KHz. This setup allows us to measure the differential reflection, that is, the pump-induced relative change of the probe reflection. It is defined as ΔR/R0 = (R − R0)/R0, where R and R0 are the probe reflection of the sample with and without the presence of the pump, respectively. This quantity can be measured as a function of the probe delay, defined as the arrival time of the probe pulse with respect to the pump pulse, and the probe position, defined as the distance between the centers of the pump and probe laser spots. All the measurements were performed at room temperature with the sample exposed in air.
3. Results and discussion
We used the 395-nm pulse (photon energy of 3.18 eV) to excite the graphene-WS2 heterostructure sample. With an energy fluence of 0.9 μJ cm−2, the pulse injects carrier densities of 3×1011 and 4.5 × 1010 cm−2 in WS2 and graphene layers, respectively, which are estimated from the absorption coefficients of these materials [13, 46]. The 625-nm probe pulse (1.98 eV), tuned near the peak of the A-exciton resonance of WS2, was used to probe the carrier dynamics in the sample. The black squares in Fig. 2(a) show the observed differential reflection signal as a function of the probe delay. The signal reaches a peak of about 2.7 × 10−3 rapidly, then decay exponentially.
Fig. 2 (a) Differential reflection signal as a function of probe delay with pump fluencs of (from top to bottom) 0.9, 0.6, 0.45, 0.3, 0.2, and 0.05 μJ cm−2, respectively. The red curves are fits of Gaussian integral (rising part) and exponential (decay part). (b) Peak values of differential reflection as a function of the pump fluence. The black line is a linear fit. (c) The decay time constants (blue) and rising time (red) deduced from the fits as a function of the pump fluence.
Previously, some of us have shown that the differential reflection signal of a probe tuned to the WS2 exciton resonance in such structures is dominated by carriers in graphene, due to screening effect of these highly mobile carriers on the Coulomb field of the excitons in WS2 [28]. It was known that in 2D TMDs, the interaction between photoexcited electrons and holes is significantly enhanced due to the reduced dielectric screening of the Coulomb field between electrons and holes, which extends to the vaccum surrounding the 2D layer. This effect results in gaint exciton binding energies in 2D TMDs [47]. In the heterostructure sample, the Coulomb field between electrons and holes forming the excitons can be effectively screened by the highly mobile photocarriers in graphene, which alters the excitonic states and its coupling to light and giving rising to the observed transient reflection. This assignment is further confirmed by the large magnitude and the ultrashort delay time of the signal observed [28]. Therefore, the evolution of the signal shown in Fig. 2 reflects the dynamics of photocarriers in graphene. We find that the rising of the signal can be fit by the integral of a Gaussian function with a width (full-width at half maximum) of about 0.36 ps. This time duration is close to the width of the cross-correlation of the pump and probe pulses of about 0.35 ps. The relatively long temporal width of the pump and probe pulses at sample is due to the dispersive elements they transmit, mainly the long microscope objective lens. The pulse-width-limited rising time of the signal indicates that the photocarriers injected in WS2 transfer to graphene on a time scale shorter than the time resolution of the measurement. The decay of the signal is signal exponential, with a time constant of about 1.2 ps, reflecting the ultrashort lifetime of photocarriers in graphene.
Next, we repeat the measurement with lower values of pump fluence. As shown in Fig. 2(a), signals of smaller magnitude and similar behavior were observed. The peak signal is found to be proportional to the pump fluence [Fig. 2(b)], and the rising and decay time constants are independent of the pump fluence [Fig. 2(c)]. These features ensure that the spatial and temporal evolution of the differential reflection signal precisely follows the spatiotemporal dynamics of photocarriers [48].
Although the time resolution of the measurement is insufficient to resolve the photocarrier transfer process, additional spatial resolution can provide information on the mechanism of this process. For this purpose, the differential reflection signal was measured as both a function of the probe delay and a function of the probe position, with a pump fluence of 0.5 μJ cm−2. The results are shown in lower-left panel of Fig. 3. The right panel shows a cross section at 0.3 ps, which has a Gaussian spatial profile, while the upper panel shows the signal as a function of the probe delay when the centers of the pump and probe spots are separated by 1 μm. To analyze the change of the profile with time, we plot in Fig. 4(a) and (b) the spatial profiles at various probe delays, which are indicated in the labels of the figure. These profiles are fit by Gaussian functions. As shown as the solid curves in Fig. 4(a) and (b), the fits are of high quality, which allow us to extract full-width at half maximum (FWHM) of the profiles accurately. As shown by the black squares in in Fig. 4(c), surprisingly, the width decreases with time. That is, the spatial profile of the photocarrier density in graphene narrows with time. We note that the initial width at −0.1 ps is close to the width of the convoluted pump and probe spot size. The narrowing is rather pronounced, with the width decreases by about 10 % of its initial value that is determined by the laser spots used in the measurement. We repeated the measurement several times with slightly different experimental conditions. the narrowing of the profile was observed each time. An example of these repeated measurements is shown by the red circles in Fig. 4(c), which was performed with a slightly smaller pump laser spot.
Fig. 3 Differential reflection signal as a function of the probe delay and probe position. The upper and right panels are line scans at a fixed position and time, respectively, as indicated in thee lower-left panel.
Fig. 4 Spatial profiles of the differential reflection signal at different probe delays before (a) and after (b) the peak time. The curves are Gaussian fits. (c) The black squares show the FWHM of the profiles shown in (a) and (b) as a function of the probe delay. The red circles are from a repeated measurement with a slightly smaller pump laser spot.
We attribute the observed phenomenon to the effect of electric field between electrons an holes on their interlayer transfer, as illustrated in Fig. 5. The tightly focused pump pulse injects electrons (e) and holes (h) in WS2 with a Gaussian spatial profile with higher density near the central region of the laser spot, as schematically illustrated in Fig. 5(a). In this heterostructure, the band offset for holes is larger than for electrons, as indicated in Fig. 1(b). Hence, holes experience a stronger driving force than the electrons, and hence are expected to transfer at a faster rate. Once holes are transferred, as shown in Fig. 5(b), the Coulomb field of the holes act on electrons [the red arrows in Fig. 5(b)]. From Poisson’s equation, the spatial profile of the field is also a Gaussian function, which is stronger near the center of the profile. Under the assumption that an electric field from graphene to WS2 could increase the transfer rate of electrons, the electrons near the the center transfer faster than those excited away from the center. As illustrated in Fig. 5(c), this effect causes the central part of the profile to grow faster than the wing of the profile, effectively decreasing the appearance width of the profile. We note that the simple model shown in Fig. 5 can only reveal qualitatively the effect observed. In fact, the hole and electron transfer processes may not be entirely separated in time. Furthermore, theoretical description of the interlayer charge transfer in van der Waals heterostructures is still under development [42–44]. Hence, a quantitative analysis of the experimental result is rather challenging. Nevertheless, the narrowing of the profile observed strongly suggests the effect of space charge field on the carrier transfer process. We also note that the conclusion on the field effect on charge transfer is not based on the assumption that holes transfer faster than electrons. If the electrons transfer first, the drag of their electric field on holes would produced the same narrowing effect.
Fig. 5 Effect of space charge field on charge transfer. (a) The initial nonuniform Gaussian distribution of electrons (e) and holes (h) in WS2 injected by the pump pulse. (b) Once holes transferred to graphene, a space charge field (red lines) is developed, with the same Gaussian profile. (c) The stronger field in the middle region can increase the transfer rate of electrons in that region, causing the appearance of narrowing of the carrier density profile.
4. Conclusions
We studied photocarrier transfer process in graphene-WS2 heterostructures by temporally and spatially resolved transient reflection measurements. Photocarriers are injected in WS2 by a tightly focused pump laser pulse, and were found to transfer to graphene on an ultrafast time scale. Surprisingly, the width of the spatial profile of photocarriers in graphene decreases with time. This counter-intuitive effect suggest that the Coulomb field of the transferred holes can effectively drag the electrons, speeding up their transfer from WS2 to graphene. Our results also suggest that an externally applied electric field can be used to control the photocarrier transfer in such heterostructures, despite the fact that the electron-hole pairs are neutral as a whole.
Funding
National Science Foundation (NSF) (DMR-1505852); National Basic Research Program of China (2016YFA0202300, 2016YFA0202302); Natural National Science Foundation (NSFC) (61527817, 61335006, 61378073); Beijing Science and Technology Committee (Z151100003315006); Initiative Postdocs Supporting Program (BX201600013).
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