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We report observations of a strong thickness dependence for charge transfer (CT) from MoSe2 to MoS2, as evidenced by transient absorption measurements. By time-resolving CT from MoSe2 monolayers (1Ls) to MoS2 flakes of varying thicknesses, including 1L, bilayer (2L), and trilayer (3L), we find that the CT time is several picoseconds in the 1L-MoSe2/3L-MoS2 heterostructure, which is much longer than that of 1L-MoSe2/1L-MoS2 and 1L-MoSe2/2L-MoS2 heterostructures. In addition, the recombination lifetime of the interlayer excitons in the 1L/3L heterostructure is several times longer than that of 1L/1L and 1L/2L heterostructures, reaching 800 ps. Furthermore, we show that a prepulse can reduce the CT time and enhance the interlayer exciton recombination in the 1L/3L heterostructure. These findings illustrate that layer thickness can be an important parameter to control the CT property of van der Waals heterostructures. These experimental results also provide important information for further refining the understanding of the physical mechanisms of CT in van der Waals heterostructures.

Monolayer transition-metal dichalcogenide crystals (TMDC) can be combined with other functional materials, such as organic molecules, to form a wide range of heterostructures with tailorable properties. Although a number of works have shown that ultrafast charge transfer (CT) can occur at organic/TMDC interfaces, conditions that would facilitate the separation of interfacial CT excitons into free carriers remain unclear. Here, time-resolved and steady-state photoemission spectroscopy are used to study the potential energy landscape, charge transfer, and exciton dynamics at the zinc phthalocyanine (ZnPc)/monolayer (ML) MoS2 and ZnPc/bulk MoS2 interfaces. Surprisingly, although both interfaces have a type-II band alignment and exhibit sub-100 fs CT, the CT excitons formed at the two interfaces show drastically different evolution dynamics. The ZnPc/ML-MoS2 behaves like typical donor-acceptor interfaces in which CT excitons dissociate into electron-hole pairs. On the contrary, back electron transfer occur at ZnPc/bulk-MoS2, which results in the formation of triplet excitons in ZnPc. The difference can be explained by the different amount of band bending found in the ZnPc film deposited on ML-MoS2 and bulk-MoS2. Our work illustrates that the potential energy landscape near the interface plays an important role in the charge separation behavior. Therefore, considering the energy level alignment at the interface alone is not enough for predicting whether free charges can be generated effectively from an interface.

The newly developed van der Waals materials allow fabrication of multilayer heterostructures. Early efforts have mostly focused on heterostructures formed by similar materials. More recently, however, attempts have been made to expand the types of materials, such as topological insulators and organic semiconductors. Here we introduce an amorphous semiconductor to the material library for constructing van der Waals heterostructures. Samples composed of 2 nm amorphous black phosphorus synthesized by pulsed laser deposition and monolayer WS2 obtained by mechanical exfoliation were fabricated by dry transfer. Photoluminescence measurements revealed that photocarriers excited in WS2 of the heterostructure transfer to amorphous black phosphorus, in the form of either energy or charge transfer, on a time scale shorter than the exciton lifetime in WS2. Transient absorption measurements further indicate that holes can efficiently transfer from WS2 to amorphous black phosphorus. However, interlayer electron transfer in either direction was found to be absent. The lack of electron transfer from amorphous black phosphorus to WS2 is attributed to the localized electronic states in the amorphous semiconductor. Furthermore, we show that a hexagonal BN bilayer can effectively change the hole transfer process.

Semiconducting molybdenum ditelluride has emerged as a promising transition-metal dichalcogenide with a number of novel properties. In particular, its bandgap in infrared range makes it an attracti...

An all-optical approach to control interlayer charge transfer and interlayer exciton dynamics is demonstrated using transition metal dichalcogenide heterostructures of ${\mathrm{MoSe}}_{2}/{\mathrm{MoS}}_{2}$ and ${\mathrm{MoSe}}_{2}/{\mathrm{WS}}_{2}$ as examples. In the three-pulse pump-probe experiments, a control pulse injects photocarriers and produces an electric field due to the interlayer charge separation. A pump pulse excites new carriers and their dynamics under the influence of the control-injected carriers and field is time-resolved by measuring differential reflectance of a third probe pulse. In ${\mathrm{MoSe}}_{2}/{\mathrm{MoS}}_{2}$, the interlayer electron transfer time from ${\mathrm{MoSe}}_{2}$ to ${\mathrm{MoS}}_{2}$ decreases with increasing the control-injected carrier density and electric field. The dipole moment of the interlayer excitons is reduced by the control pulse, which can be utilized for ultrafast and all-optical control of interlayer excitons. The recombination of the interlayer excitons is enhanced by the control pulse, which increases the spatial overlap of the electron and hole wave functions. The effect of the control pulse on the interlayer excitons is confirmed in the ${\mathrm{MoSe}}_{2}/{\mathrm{WS}}_{2}$ heterostructure, although its effect on the electron transfer time is not resolvable. These results reveal useful information to understand and control interlayer charge transfer dynamics and provide tools for ultrafast manipulation of electrons in van der Waals heterostructures.

We report three-pulse photodope-pump-probe measurements on photocarrier dynamics in semiconducting transition metal dichalcogenide monolayers of ${\mathrm{MoS}}_{2}$, ${\mathrm{WS}}_{2}$, ${\mathrm{MoSe}}_{2}$, and ${\mathrm{WSe}}_{2}$. The samples are fabricated by metal-organic chemical vapor deposition and mechanical exfoliation techniques and characterized by photoluminescence spectroscopy. In the time-resolved measurement, the samples are first photodoped by a prepulse, which injects background photocarriers of various densities. A pump pulse then injects photocarriers, whose dynamics is monitored by measuring a differential reflection of a time-delayed probe pulse. We found that the ultrafast decay component of the differential reflection signal, which has been widely reported before, shows minimal dependence on the background exciton density. This observation shows that a previously suggested carrier-trapping model cannot account for this component. The results thus further support an exciton-formation model that was previously proposed based on spectroscopic evidence.

The newly developed van der Waals materials allow fabrication of multilayer heterostructures. Early efforts have mostly focused on heterostructures formed by similar materials. More recently, however, attempts have been made to expand the types of materials, such as topological insulators and organic semiconductors. Here we introduce an amorphous semiconductor to the material library for constructing van der Waals heterostructures. Samples composed of 2 nm amorphous black phosphorus synthesized by pulsed laser deposition and monolayer WS

Forming van der Waals multilayer structures with two-dimensional materials is a promising new method for material discovery. The weak van der Waals interlayer interaction without atomic correspondence relaxes lattice matching requirement and allows formation of high-quality interfaces with virtually any combination of two-dimensional materials. However, the weak nature of the van der Waals interaction also makes it challenging to harness emergent properties of such multilayer materials. Previous studies have indicated that in transition metal dichalcogenide bilayer heterostructures, the interlayer charge and energy transfer is highly efficient. Therefore, it is important to understand interlayer coupling in these materials and its role on charge and energy transfer. Here we show that in a MoSe2/WSe2/WS2 trilayer, the interlayer coupling is strong enough to form layer-coupled states in the conduction band with the electron wave function extends to all three layers. Density functional theory calculations reveal that the layer-coupled states in Q valley are about 0.1 eV below the individual monolayer states in K valley, which is consistent with photoluminescence measurements. Transient absorption measurements show that these layer-coupled states provide a channel for ultrafast interlayer charge transfer between the top WS2 and the bottom MoSe2 layers. In this process, electrons from the K valley of the individual monolayers are scattered to the layer-coupled states in Q valley. Such a partial charge transfer allows formation of partial-indirect excitons with the holes in one monolayer while electrons shared by three layers. The formation of layer-coupled states is promising for harnessing emergent properties of transition metal dichalcogenide multilayer heterostructures. Our findings also provide new ingredient to understand charge and energy transfer in transition metal dichalcogenide heterobilayers, as the layer-coupled states can play important roles in the efficient transfer observed in these systems.

We report an experimental study on charge transfer properties of mixed-dimensional heterostructures formed by zero-dimensional PbS quantum dots and two-dimensional transition metal dichalcogenides. Monolayers of ${\mathrm{MoSe}}_{2}$ and ${\mathrm{MoS}}_{2}$ were fabricated by mechanical exfoliation and chemical vapor deposition techniques, respectively. PbS quantum dots with diameters of 2.3 and 5 nm were synthesized by a hot-injection method and characterized by optical absorption spectroscopy and ultraviolet photoemission spectroscopy. The quantum dots were deposited on the ${\mathrm{MoS}}_{2}$ and ${\mathrm{MoSe}}_{2}$ monolayers to form heterostructures. Photoluminescence and transient absorption measurements were performed on the heterostructures as well as individual materials to reveal their photocarrier dynamics. We found that the holes excited in ${\mathrm{MoSe}}_{2}$ can efficiently transfer to both 2.3- and 5-nm PbS quantum dots, while electrons in these quantum dots cannot transfer to ${\mathrm{MoSe}}_{2}$. Similar charge transfer properties were observed between ${\mathrm{MoS}}_{2}$ and the 5-nm PbS quantum dots, while no charge transfer was observed between ${\mathrm{MoS}}_{2}$ and the 2.3-nm quantum dots. These results provide useful information for understanding the physical mechanism of charge transfer in mixed-dimensional heterostructures and for developing PbS quantum-dot-based mixed-dimensional materials.

We report the properties of hybrid deposited iron pyrite (FeS2) thin films applied as the back contact interface layers of CdS/CdTe solar cells. The hybrid deposition process for FeS2 optimized in this study relies on DC magnetron sputtering of iron with simultaneous thermal evaporation of sulfur. We have fabricated solar cells incorporating CdS/CdTe window/absorber layers sputter-deposited onto commercial transparent conducting oxide coated glass and have compared the performance of devices incorporating the new FeS2/Cu/Au back contacts with that of standard devices incorporating Cu/Au back contacts. Considering our best devices of each type, the inclusion of the FeS2 thin film as a hole transport layer has improved the open circuit voltage VOC by 2.1%, reaching 817 mV, and the fill-factor FF by 8.3% relative, reaching 74.7%, in comparison with devices omitting the FeS2 layer. Under standard test conditions of 100 mA/cm2 simulated AM1.5G and 25 °C, devices utilizing the FeS2 hole transport layer have shown a conversion efficiency η as high as 13.3% – a relative increase in η of ~10% over our current laboratory standard back contact. The attained FF exceeds previous results for high efficiency sputter-deposited CdS/CdTe solar cells.

Monolayer transition-metal dichalcogenide crystals (TMDC) can be combined with other functional materials, such as organic molecules, to form a wide range of heterostructures with tailorable properties. Although a number of works have shown that ultrafast charge transfer (CT) can occur at organic/TMDC interfaces, conditions that would facilitate the separation of interfacial CT excitons into free carriers remain unclear. Here, time-resolved and steady-state photoemission spectroscopy are used to study the potential energy landscape, charge transfer, and exciton dynamics at the zinc phthalocyanine (ZnPc)/monolayer (ML) MoS

We observed ultrafast charge transfer between distorted $1T\text{\ensuremath{-}}{\mathrm{ReS}}_{2}$ with anisotropic in-plane electronic and optical properties and $2H\text{\ensuremath{-}}{\mathrm{WS}}_{2}$ that is in-plane isotropic. Heterostructures of monolayer ${\mathrm{ReS}}_{2}$/monolayer ${\mathrm{WS}}_{2}$ and bilayer ${\mathrm{ReS}}_{2}$/monolayer ${\mathrm{WS}}_{2}$ were fabricated by mechanical exfoliation and dry transfer techniques. Significant photoluminescence quenching of ${\mathrm{WS}}_{2}$ in the heterostructures indicates efficient charge transfer. In femtosecond transient absorption measurements, it was found that holes injected in monolayer or bilayer ${\mathrm{ReS}}_{2}$ transfer to ${\mathrm{WS}}_{2}$ on a timescale that is shorter than the time resolution of the measurement. This observation provides evidence that the holes are delocalized in bilayer ${\mathrm{ReS}}_{2}$, revealing strong van der Waals interlayer couplings. These results also show that ${\mathrm{ReS}}_{2}$ and ${\mathrm{WS}}_{2}$ form type-II heterostructures with excellent charge transfer properties.

We report a combined experimental and computational study on the charge transfer properties of heterostructures formed by monolayer 1T′-ReS2 and fluorinated zinc phthalocyanine, F8ZnPc. Two-dimensional (2D) ReS2 monolayer flakes were exfoliated from bulk crystals, and an F8ZnPc film with a thickness of 4 nm was thermally deposited on ReS2. Density functional theory shows that the two materials form a type-II band alignment. In transient absorption measurements, photocarriers are selectively excited in ReS2 and monitored in F8ZnPc. We found that holes in ReS2 can transfer to F8ZnPc on a timescale shorter than 0.35 ps. The transferred holes have a long lifetime in F8ZnPc on the order of 1 ns, confirming the lack of electron transfer. The efficient charge transfer from a 1T′ transition metal dichalcogenide monolayer to an organic semiconductor illustrates the feasibility of developing 2D/organic heterostructures with in-plane anisotropic electronic and optoelectronic properties.

The recently discovered two-dimensional materials can be used to fabricate multilayer heterostructures. Interlayer charge transfer is a key process in such heterostructures as it can enable emergent optoelectronic properties. Efficient interlayer charge transfer in van der Waals heterostructures has been observed by femtosecond transient absorption and steady-state optical spectroscopy measurements, based on measuring the interlayer carrier distribution. Here, we show that a second harmonic generation process allows direct probing of the electric field induced by the charge transfer. An ultrashort laser pulse was used to excite electrons and holes in a MoS2/WS2 heterostructure. The separation of the electrons and holes from the two monolayers generates an electric field, which enables the generation of the second harmonic of an incident fundamental pulse. We further studied the time evolution of this electric field by measuring the second harmonic signal as a function of the time delay between the pump and the fundamental pulses. The result agrees well with the dynamics revealed by a transient absorption measurement. These results provide direct evidence of interlayer charge transfer and demonstrate an all-optical method of studying charge transfer and induced electric fields in two-dimensional materials. Furthermore, this effect, if large enough, could be utilized in optical devices based on 2D heterostructures with nonlinear optical responses controllable by interlayer charge transfer.

We report a combined theoretical and experimental study on photocarrier dynamics in monolayer phosphorene and bulk black phosphorus. Samples of monolayer phosphorene and bulk black phosphorus were fabricated by mechanical exfoliation, identified according to their reflective contrasts, and protected by covering them with hexagonal boron nitride layers. Photocarrier dynamics in these samples was studied by an ultrafast pump-probe technique. The photocarrier lifetime of monolayer phosphorene was found to be about 700 ps, which is about 9 times longer than that of bulk black phosphorus. This trend was reproduced in our calculations based on ab initio nonadiabatic molecular dynamics combined with time-domain density functional theory in the Kohn-Sham representation, and can be attributed to the smaller bandgap and stronger nonadiabatic coupling in bulk. The transient absorption response was also found to be dependent on the sample orientation with respect to the pump polarization, which is consistent with the previously reported anisotropic absorption of phosphorene. In addition, an oscillating component of the differential reflection signal at early probe delays was observed in the bulk sample and was attributed to the layer-breathing phonon mode with an energy of about 1 meV and a decay time of about 1.35 ps. These results provide valuable information for application of monolayer phosphorene in optoelectronics.

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 tuned 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 two-dimensional stacked layered materials, where their properties are tailored monolayer by monolayer. By arranging ${\mathrm{WSe}}_{2},{\mathrm{MoSe}}_{2},{\mathrm{WS}}_{2}$, and ${\mathrm{MoS}}_{2}$ 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 ${\mathrm{WSe}}_{2}$ monolayer, a ${\mathrm{WSe}}_{2}\text{\ensuremath{-}}{\mathrm{MoSe}}_{2}$ bilayer, a ${\mathrm{WSe}}_{2}\text{\ensuremath{-}}{\mathrm{MoSe}}_{2}\text{\ensuremath{-}}{\mathrm{WS}}_{2}$ trilayer, and a ${\mathrm{WSe}}_{2}\text{\ensuremath{-}}{\mathrm{MoSe}}_{2}\text{\ensuremath{-}}{\mathrm{WS}}_{2}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ 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.

We report the properties of sputtered/co-evaporated thin film iron pyrite as the back contact interface layer of CdS/CdTe solar cells. In our study we have compared the performance of sputter-deposited CdTe devices which utilized either a Cu/Au contact, or an FeS 2 /Cu/Au contact. Comparing our best devices of each type, the FeS 2 contact layer has improved the V OC by ∼16 mV, to 816 mV, and improved the FF by ∼4.7% absolute to 73.7%, with respect to devices omitting the FeS 2 layer. Under Standard Test Conditions of 100 mW/cm and 25 °C, devices utilizing the FeS 2 back contact have shown a relative increase in the power conversion efficiency by ∼7.5%, with a best device efficiency of 12.9%.

Local density of state (LDOS), current–voltage characteristic ( I – V characteristic) and differential conductance (DC) of pure and defected zigzag and armchair graphene nanoribbons quantum dots (GNRQDs) are investigated. We found that for the pure systems of armchair GNRQDs, the DC has oscillation feature where these oscillations could be explained by 1D characteristic of atomic wires. In our samples, these wires constructed from sites with the same symmetry along the GNRQD length. Some of these wires have metallic features while some of them show semiconducting features. The zigzag GNRQDs have approximately ohmic behavior. Also it is found that the defected DC significantly depend on vacancy position.

High quality and stable electrical contact between metal and two-dimensional materials, such as transition metal dichalcogenides, is a necessary requirement that has yet to be achieved in order to successfully exploit the advantages that these materials offer to electronics and optoelectronics. MoTe2, owing to its phase changing property, can potentially offer a solution. A recent study demonstrated that metallic phase of MoTe2 connects its semiconducting phase with very low resistance. To utilize this property to connect other two-dimensional materials, it is important to achieve efficient charge transfer between MoTe2 and other semiconducting materials. Using MoS2 as an example, we report ultrafast and efficient charge transfer between MoTe2 and MoS2 monolayers. In the transient absorption measurements, an ultrashort pump pulse is used to selectively excite electrons in MoTe2. The appearance of the excited electrons in the conduction band of MoS2 is monitored by using a probe pulse that is tuned to the resonance of MoS2. We found that electrons transfer to MoS2 on a time scale of at most 0.3 ps. The transferred electrons give rise to a large transient absorption signal at both A-exciton and B-exciton resonances due to the screening effect. We also observed ultrafast transfer of holes from MoS2 to MoTe2. Our results suggest the feasibility of using MoTe2 as a bridge material to connect MoS2 and other transition metal dichalcogenides, and demonstrate a new transition metal dichalcogenide heterostructure involving MoTe2, which extends the spectral range of such structures to infrared.