Carrier multiplication in van der Waals layered transition metal dichalcogenides (original) (raw)
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Highly Efficient Carrier Multiplication in van der Waals layered Materials
arXiv: Mesoscale and Nanoscale Physics, 2018
Carrier multiplication (CM), a photo-physical process to generate multiple electron-hole pairs by exploiting excess energy of free carriers, is explored for efficient photovoltaic conversion of photons from the blue solar band, predominantly wasted as heat in standard solar cells. Current state-of-the-art approaches with nanomaterials have demonstrated improved CM but are not satisfactory due to high energy loss and inherent difficulties with carrier extraction. Here, we report ultra-efficient CM in van der Waals (vdW) layered materials that commences at the energy conservation limit and proceeds with nearly 100% conversion efficiency. A small threshold energy, as low as twice the bandgap, was achieved, marking an onset of quantum yield with enhanced carrier generation. Strong Coulomb interactions between electrons confined within vdW layers allow rapid electron-electron scattering to prevail over electron-phonon scattering. Additionally, the presence of electron pockets spread over...
Interlayer excitonic states in MoSe2/MoS2 van der Waals heterostructures
Physical Review B
In the present work, we report different interlayer excitonic states of an aligned MoSe 2 /MoS 2 incommensurate van der Waals (vdW) heterostructure (HS). The HS was fabricated by stacking chemical vapor deposited monolayers, and it was studied using photoluminescence (PL) measurements. We observed the emergence of two low-energy peaks in the PL spectrum of the HS measured at 100 K, which were absent in the constituent monolayers. The orbital resolved electronic band structure and the optical absorption considering the electronhole interaction for these HSs were calculated using first-principles density functional theory simulations, which showed energy band hybridization and the presence of interlayer excitons (ILEs). Based on these observations, the peak at ∼1.57 eV was assigned to a momentum direct ILE, while the other peak at ∼1.35 eV showed feeble emission intensity and was assigned to a momentum indirect ILE. The emergence of both of these excitonic peaks in the HS PL spectrum can be attributed to the formation of a spatially periodic moiré potential at a nanometer length scale resulting in hybridization. Our results can help to understand the physics of ILEs and to engineer vdW HSs for efficient optoelectronic devices.
Light Generation and Harvesting in a van der Waals Heterostructure
ACS Nano, 2014
Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS 2 or WSe 2 have been proposed as promising channel materials for field-effect transistors (FETs). Their high mechanical flexibility, stability and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS 2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS 2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from
Carrier Multiplication via Photocurrent Measurements in Dual-Gated MoTe_2
arXiv: Mesoscale and Nanoscale Physics, 2020
Although van der Waals layered transition metal dichalcogenides from transient absorption spectroscopy have successfully demonstrated an ideal carrier multiplication (CM) performance with an onset of nearly 2Eg,interpretation of the CM effect from the optical approach remains unresolved owing to the complexity of many-body electron-hole pairs. We demonstrate the CM effect through simple photocurrent measurements by fabricating the dual-gate P-N junction of a MoTe2 film on a transparent substrate. Electrons and holes were efficiently extracted by eliminating the Schottky barriers in the metal contact and minimizing multiple reflections. The photocurrent was elevated proportionately to the excitation energy. The boosted quantum efficiency confirms the multiple electron-hole pair generation of >2Eg, consistent with CM results from an optical approach, pushing the solar cell efficiency beyond the Shockley-Queisser limit.
2021
Two dimensional (2D) van der Waals heterostructures (vdWHs) have their unique potential in facilitating the stacking of layers of different 2D materials for optoelectronic devices with superior characteristics at a reduced cost. However, the fabrication of large area all-2D heterostructures is still challenging towards realizing practical devices. In the present work, we have demonstrated a rapid yet simple, impurity free and highly efficient sonication-assisted chemical exfoliation approach to synthesize hybrid vdWHs based on 2D molybdenum disulphide (MoS$_2$) and tungsten disulphide (WS$_2$), with high yield. Microscopic and spectroscopic studies have confirmed the successful exfoliation of layered 2D materials and formation of their hybrid heterostructure. The co-existence of 2D MoS2 and WS2 in the vdW hybrid is established by optical absorption and Raman shift measurements along with their chemical stiochiometry determined by X-ray photoelectron spectroscopy. The spectral respon...
Interlayer coupling effect in van der Waals heterostructures of transition metal dichalcogenides
Frontiers of Physics, 2020
Van der Waals (vdW) heterobilayers formed by two-dimensional (2D) transition metal dichalcogenides (TMDCs) created a promising platform for various electronic and optical properties. ab initio band results indicate that the band offset of type-II band alignment in TMDCs vdW heterobilayer could be tuned by introducing Janus WSSe monolayer, instead of an external electric field. On the basis of symmetry analysis, the allowed interlayer hopping channels of TMDCs vdW heterobilayer were determined, and a four-level k•p model was developed to obtain the interlayer hopping. Results indicate that the interlayer coupling strength could be tuned by interlayer electric polarization featured by various band offsets. Moreover, the difference in the formation mechanism of interlayer valley excitons in different TMDCs vdW heterobilayers with various interlayer hopping strength was also clarified.
High-performance optoelectronic devices based on van der Waals vertical MoS2/MoSe2 heterostructures
Nano Research, 2020
Monolayer MoS 2 is a direct band gap semiconductor with large exciton binding energy, which is a promising candidate for the application of ultrathin optoelectronic devices. However, the optoelectronic performance of monolayer MoS 2 is seriously limited to its growth quality and carrier mobility. In this work, we report the direct vapor growth and the optoelectronic device of verticallystacked MoS 2 /MoSe 2 heterostructure, and further discuss the mechanism of improved device performance. The optical and high-resolution atomic characterizations demonstrate that the heterostructure interface is of high-quality without atomic alloying. Electrical transport measurements indicate that the heterostructure transistor exhibits a high mobility of 28.5 cm 2 /(V•s) and a high on/off ratio of 10 7. The optoelectronic characterizations prove that the heterostructure device presents an enhanced photoresponsivity of 36 A/W and a remarkable detectivity of 4.8 × 10 11 Jones, which benefited from the interface induced built-in electric field and carrier dependent Coulomb screening effect. This work demonstrates that the construction of two-dimensional (2D) semiconductor heterostructures plays a significant role in modifying the optoelectronic device properties of 2D materials.
Separating electrons and holes by monolayer increments in van der Waals heterostructures
Physical Review Materials
Since the discovery of graphene and its outstanding chemical, optical, and mechanical properties, other layered materials have been fiercely hunted for throughout various techniques. Thanks to their van der Waals interaction, acting as weak glue, different types of layered materials with mismatched lattices can be stacked with high quality interfaces. The properties of the resulting multilayer structures can be tune by choice of the materials, layer thicknesses, and sequence in which they are arranged. This opens the possibility for a large array of applications across many different fields. Here we present a systematic study with twodimensional stacked layered materials, where their properties are tailored monolayer by monolayer. By arranging WSe 2 , MoSe 2 , WS 2 , and MoS 2 monolayers in predetermined sequences, that are predicted to have a ladder band alignment in both the conduction and valence bands, we separate electrons and holes between the two utmost layers by monolayer increments. The samples studied are a WSe 2 monolayer, a WSe 2-MoSe 2 bilayer, a WSe 2-MoSe 2-WS 2 trilayer, and a WSe 2-MoSe 2-WS 2-MoS 2 four-layer. We observe an increase in absorbance, a decrease in photoluminescence, a variation in interlayer charge transfer, and photocarrier lifetimes that are extended up to a few nanoseconds as additional layers were added. With these results, we demonstrate that van der Waals stacked two-dimensional materials can form effective complex stacks and are promising platforms for fabricating ultrathin and flexible optoelectronics. 1
Distinct Photoluminescence in Multilayered van der Waals Heterostructures of MoS 2 /WS 2 /ReS
van der Waals heterostructures of (TMD L ¼ 1 /BN L ¼ 1-4 /TMD L ¼ 1/ BN L ¼ 1-4 ), 6 [TMD ¼ MoS 2 , WS 2 , and ReS 2 ] are grown on c-plane sapphire substrate by 7 pulsed laser deposition (PLD) under slow kinetic condition. The heterostruc-8 ture systems show strong emission around 2.3 eV and subsidiary peaks 9 around 2.8, 1.9, 1.7, and 1.5 eV. BN and transition metal dichalcogenides 10 (TMDs) form type-I heterojunction and the emission peaks observed are 11 explained in terms of various band to band recombination processes and 12 considering relative orientation of Brillouin Zones. The emission peak around 13 2.3 eV is promising for solar and photovoltaic applications. The observation is 14 almost similar for three different heterostructure systems.