Negative Differential Photoconductance as a Signature of Nonradiative Energy Transfer in van der Waals Heterojunction
The physical proximity of layered materials in their van der Waals heterostructures (vdWhs) aids interfacial phenomena such as charge transfer (CT) and energy transfer (ET). Besides providing fundamental insights, CT and ET also offer routes to engineer optoelectronic properties of vdWhs. For example, harnessing ET in vdWhs can help to overcome the limitations of optical absorption imposed by the ultra-thin nature of layered materials and thus provide an opportunity for in situ enhancement of quantum efficiency for light-harvesting and sensing applications. While several spectroscopic studies on vdWhs probed the dynamics of CT and ET, the possible contribution of ET in the photocurrent generation remains largely unexplored. In this work, we investigate the role of nonradiative energy transfer (NRET) in the photocurrent through a vertical vdWh of SnSe2/MoS2/TaSe2. We observe an unusual negative differential photoconductance (NDPC) arising from the existence of NRET across the SnSe2/MoS2 junction. Modulation of the NRET-driven NDPC characteristics with optical power results in a striking transition of the photocurrent’s power law from a sublinear to a superlinear regime. Our observations reveal the nontrivial influence of ET on the photoresponse of vdWhs, which offer insights to harness ET in synergy with CT for vdWh based next-generation optoelectronics.
More details: ACS Nano, 15, 10, 16432–16441, 2021.
Spectrally Tunable, Large Raman Enhancement from Nonradiative Energy Transfer in the van der Waals Heterostructure
Raman enhancement techniques are essential for fundamental studies in light–matter interactions and find widespread application in microelectronics, biochemical sensing, and clinical diagnosis. Two-dimensional (2D) materials and their van der Waals heterostructures (vdWHs) are emerging rapidly as potential platforms for Raman enhancement. Here, we experimentally demonstrate a new technique of Raman enhancement driven by nonradiative energy transfer (NRET), achieving a 10-fold enhancement in the Raman intensity in a vertical vdWH comprising of a monolayer transition metal dichalcogenide (1L-TMD) placed on a multilayer SnSe2. Consequently, several weak Raman peaks become visible, which are otherwise imperceptible. We also show a strong modulation of the enhancement factor by tuning the spectral overlap between the 1L-TMD and SnSe2 through temperature variation, and the results are in remarkable agreement with a Raman polarizability model capturing the effect of NRET. The observed NRET-driven Raman enhancement is a novel mechanism that has not been experimentally demonstrated thus far and is distinct from conventional surface (SERS), tip (TERS), or interference enhanced Raman scattering (IERS) mechanisms that are driven solely by charge transfer or electric field enhancement. The mechanism can also be used in synergy with plasmonic nanostructures to achieve additional selectivity and sensitivity beyond hot spot engineering for applications like molecular detection using 2D/molecular hybrids. Our results open new avenues for engineering Raman enhancement techniques coupling the advantages of uniform enhancement accessible across a wide junction area in vertical vdWHs.
More details: ACS Photonics, 7, 2, 519–527, 2020.
Origin of selective enhancement of sharp defect emission lines in monolayer WSe2 on rough metal substrate
The defect states in atomically thin layers of transition metal dichalcogenides are promising candidates for single photon emission. However, the brightness of such quantum emission is often weak and is accompanied by undesirable effects such as spectral diffusion and strong background emission. By placing a monolayer WSe2 directly on a rough gold substrate, here, we show a selective enhancement of sharp defect-bound exciton peaks, coupled with a suppressed spectral diffusion and strong quenching of background luminescence. By combining the experimental data with detailed electromagnetic simulations, we reveal that such selective luminescence enhancement originates from a combination of the Purcell effect and a wavelength dependent increment of the excitation electric field at the tips of tall rough features, coupled with a localized strain-induced exciton funneling effect. Notably, insertion of a thin hexagonal boron nitride sandwich layer between WSe2 and the Au film results in a strong enhancement of the background luminescence, obscuring the sharp defect peaks. The findings demonstrate a simple strategy of using monolayer WSe2 supported by a patterned metal film that offers a possibility of achieving quantum light sources with high purity, high brightness, and suppressed spectral diffusion.
More details: Journal of Applied Physics, 127, 073105, 2020.
Highly Sensitive, Fast Graphene Photodetector with Responsivity >106 A/W Using Floating Quantum Well Gate
Graphene, owing to its zero bandgap electronic structure, is promising as an absorption material for ultra-wideband photodetection applications. However, graphene-absorption based detectors inherently suffer from poor responsivity due to weak absorption and fast photocarrier recombination, limiting their viability for low intensity light detection. Here we use a graphene/WS2/MoS2 vertical heterojunction to demonstrate a highly sensitive photodetector, where the graphene layer serves dual purpose, namely as the light absorption layer, and also as the carrier conduction channel, thus maintaining the broadband nature of the photodetector. A fraction of the photoelectrons in graphene encounter ultra-fast inter-layer transfer to a floating monolayer MoS2 quantum well providing strong quantum confined photogating effect. The photodetector shows a responsivity of 4.4 × 106 A/W at 30 fW incident power, outperforming photodetectors reported till date where graphene is used as light absorption material by several orders. In addition, the proposed photodetector exhibits an extremely low noise equivalent power (NEP) of <4 fW/√Hz and a fast response (∼ milliseconds) with zero reminiscent photocurrent. The findings are attractive towards the demonstration of graphene-based highly sensitive, fast, broadband photodetection technology.
More details: ACS Applied Materials & Interfaces, 11, 30010, 2019.
Strong Single- and Two-Photon Luminescence Enhancement by Nonradiative Energy Transfer across Layered Heterostructure
The strong light–matter interaction in monolayer transition metal dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their direct band gap nature and the ultrafast radiative decay of the strongly bound excitons these materials host. However, the impeded amount of light absorption imposed by the ultrathin nature of the monolayers impairs their viability in photonic applications. Using a layered heterostructure of a monolayer TMD stacked on top of strongly absorbing, nonluminescent, multilayer SnSe2, we show that both single-photon and two-photon luminescence from the TMD monolayer can be enhanced by a factor of 14 and 7.5, respectively. This is enabled through interlayer dipole–dipole coupling induced nonradiative Förster resonance energy transfer (FRET) from SnSe2 underneath, which acts as a scavenger of the light unabsorbed by the monolayer TMD. The design strategy exploits the near-resonance between the direct energy gap of SnSe2 and the excitonic gap of monolayer TMD, the smallest possible separation between donor and acceptor facilitated by van der Waals heterojunction, and the in-plane orientation of dipoles in these layered materials. The FRET-driven uniform single- and two-photon luminescence enhancement over the entire junction area is advantageous over the local enhancement in quantum dot or plasmonic structure integrated 2D layers and is promising for improving quantum efficiency in imaging, optoelectronic, and photonic applications.
More details: ACS Nano, 13, 4795, 2019.
Self-Powered, Highly Sensitive, High Speed Photodetection Using ITO/WSe2/SnSe2 Vertical Heterojunction
2-D transition metal di-chalcogenides are the promising candidates for ultralow intensity photodetection. However, the performance of these photodetectors is usually limited by ambience induced rapid performance degradation and long-lived charge trapping induced slow response with a large persistent photocurrent when the light source is switched off. Here, we demonstrate an indium tin oxide (ITO)/WSe2/SnSe2-based vertical double heterojunction photoconductive device where the photoexcited hole is confined in the double barrier quantum well, whereas the photo-excited electron can be transferred to either the ITO or the SnSe2 layer in a controlled manner. The intrinsically short transit time of the photoelectrons in the vertical double heterojunction helps us to achieve high responsivity in excess of 1100 A/W and fast transient response time on the order of 10 µs. A large built-in field in the WSe2 sandwich layer results in photodetection at zero external bias allowing a self-powered operation mode. The encapsulation from the top and bottom protects the photo-activeWSe2 layer from ambience induceddetrimental effects and substrate induced trapping effects helping us to achieve repeatable characteristics over many cycles.
More details: IEEE Transactions on Electron Devices, Special Issue on 2D Materials for Electronic, Optoelectronic and Sensor Devices, 65, 4141, 2018.
Substrate effects in high gain, low operating voltage SnSe2 photoconductor
High gain photoconductive devices find wide spread applications in low intensity light detection. Ultra-thin layered materials have recently drawn a lot of attention from researchers in this regard. However, in general, a large operating voltage is required to obtain large responsivity in these devices. In addition, the characteristics are often confounded by substrate induced trap effects. We study multi-layer SnSe2 based photoconductive devices using two different structures: (1) SiO2 substrate supported inter-digitated electrode (IDE), and (2) suspended channel. The IDE device exhibits a superior low voltage performance over existing literature on planar 2D structures. However, the responsivity reduces by more than two orders of magnitude, while the transient response improves for the suspended device—providing insights into the critical role played by the channel-substrate interface in the gain mechanism. The results, on one hand, are promising for highly sensitive photoconductive applications consuming ultra-low power, and on the other hand, show a generic methodology that could be applied to other layered material based photoconductive devices as well for extracting the intrinsic behavior.
More details: Nanotechnology, 29, 035205, 2018.
Asymmetrically Encapsulated vertical ITO/MoS2/Cu2O photodetector with ultra-high sensitivity
Strong light absorption, coupled with moderate carrier transport properties, makes 2D layered transition metal dichalcogenide semiconductors prom-ising candidates for low intensity photodetection applications. However, the performance of these devices is severely bottlenecked by slow response with persistent photocurrent due to long lived charge trapping, and nonreliable characteristics due to undesirable ambience and substrate effects. Here ultrahigh speciﬁc detectivity (D*) of 3.2 × 1014 Jones and responsivity (R) of 5.77 × 104 A W-1 are demonstrated at an optical power density (Pop) of 0.26 W m-2 and external bias (Vext) of −0.5 V in an indium tin oxide/MoS2/copper oxide/Au vertical multi-heterojunction photodetector exhibiting small carrier transit time. The active MoS2 layer being encapsulated by carrier collection layers allows us to achieve repeatable characteristics over large number of cycles with negligible trap assisted persistent photocurrent. A large D* > 1014 Jones at zero external bias is also achieved due to the built-in ﬁeld of the asymmetric photodetector.
More details: Small, 14, 1702066, 2018.
Photoresponse of Atomically Thin MoS2 Layers and Their Planar Heterojunctions
MoS2 monolayers exhibit excellent light absorption and large thermoelectric power, which are, however, accompanied by a very strong exciton binding energy – resulting in complex photoresponse characteristics. We study the electrical response to scanning photo-excitation on MoS2 monolayer (1L) and bilayer (2L) devices, and also on monolayer/bilayer (1L/2L) planar heterojunction and monolayer/few-layer/multilayer (1L/FL/ML) planar double heterojunction devices to unveil the intrinsic mechanisms responsible for photocurrent generation in these materials and junctions. A strong photoresponse modulation is obtained by scanning the position of the laser spot, as a consequence of controlling the relative dominance of a number of layer dependent properties, including (i) the photoelectric effect (PE), (ii) the photothermoelectric effect (PTE), (iii) the excitonic effect, (iv) hot photo-electron injection from metal, and (v) carrier recombination. The monolayer and bilayer devices show a peak photoresponse when the laser is focused at the source junction, while the peak position shifts to the monolayer/few-layer junction in the heterostructure devices. The photoresponse is found to be dependent on the incoming light polarization when the source junction is illuminated, although the polarization sensitivity drastically reduces at the monolayer/few-layer heterojunction. Finally, we investigate the laser position dependent transient response of the photocurrent to reveal that trapping of carriers in SiO2 at the source junction is a critical factor to determine the transient response in 2D photodetectors, and also show that, by a systematic device design, such trapping can be avoided in the heterojunction devices, resulting in a fast transient response. The insights obtained will play an important role in designing a fast 2D TMD based photodetector and related optoelectronic and thermoelectric devices.
More details: Nanoscale, 8, 15213-15222, 2016.