Ultrafast Charge-Transfer Dynamics in Twisted MoS2/WSe2 Heterostructures.

Two-dimensional transition metal dichalcogenides offer a fascinating platform for creating van der Waals heterojunctions with exciting physical properties. Because of their typical type-II band alignment, photoexcited electrons and holes can separate via interfacial charge transfer. Furthermore, the relative crystallographic alignment of the individual layers in these heterostructures represents an important degree of freedom. Based on both effects, various fascinating ideas for applications in optoelectronics and valleytronics have been suggested. Despite its utmost importance for the design and efficiency of potential devices, the nature and the dynamics of ultrafast charge transfer are not yet well understood. This is mainly because the charge transfer can be surprisingly fast, usually faster than the temporal resolution of previous experimental approaches. Here, we apply time- and polarization-resolved second-harmonic imaging microscopy to investigate the charge-transfer dynamics for three MoS2/WSe2 heterostructures with different stacking angles at a previously unattainable time resolution of ≈10 fs. For 1.70 eV excitation energy, electron transfer from WSe2 to MoS2 is found to depend considerably on the stacking angle with the fastest transfer time observed to be as short as 12 fs. At 1.85 eV excitation energy, ultrafast hole transfer from MoS2 to hybridized states at the Γ-point and to the K-points of WSe2 has to be considered. Surprisingly, the corresponding decay dynamics show only a minor stacking-angle dependence indicating that radiative recombination of momentum-space indirect Γ-K excitons becomes the dominant decay route for all samples.

Directional ultrafast charge transfer in a WSe2/MoSe2 heterostructure selectively probed by time-resolved SHG imaging microscopy.

Heterostructures of two-dimensional transition metal dichalcogenides (TMD) have shown promise for various optoelectronic and novel valleytronic applications. Due to their type-II band alignment, photoexcited electrons and holes can separate into different layers through ultrafast charge transfer. While this charge-transfer process is critical for potential applications, the underlying mechanisms still remain elusive. Here, we demonstrate for a rotationally mismatched WSe2/MoSe2 heterostructure that directional ultrafast charge transfer between the layers becomes accessible by time-resolved optical second-harmonic generation. By tuning the photon energy of the pump pulse, one of the two materials is resonantly excited, whereas the polarization of the probe pulse can be optimized to selectively detect the charge transfer into the other material. This allows us to explore the interlayer hole transfer from the WSe2 into the MoSe2 layer and vice versa, which appears within a few hundred femtoseconds via hybridized intermediate states at the Γ-point. Our approach enables systematic investigations of the charge transfer in dependence of the rotational layer mismatch in TMD heterostructures.

Second-harmonic imaging microscopy for time-resolved investigations of transition metal dichalcogenides.

Two-dimensional transition metal dichalcogenides (TMDC) have shown promise for various applications in optoelectronics and so-called valleytronics. Their operation and performance strongly depend on the stacking of individual layers. Here, optical second-harmonic generation (SHG) in imaging mode is shown to be a versatile tool for systematic time-resolved investigations of TMDC monolayers and heterostructures in consideration of the material's structure. Large sample areas can be probed without the need of any mapping or scanning. By means of polarization dependent measurements, the crystalline orientation of monolayers or the stacking angles of heterostructures can be evaluated for the whole field of view. Pump-probe experiments then allow to correlate observed transient changes of the second-harmonic response with the underlying structure. The corresponding time-resolution is virtually limited by the pulse duration of the used laser. As an example, polarization dependent and time-resolved measurements on mono- and multilayer MoS2flakes grown on a SiO2/Si(001) substrate are presented.

Controlling an SN 2 Reaction by Electronic and Vibrational Excitation: Tip-Induced Ether Cleavage on Si(001).

Controlling chemical reactions beyond thermally activated reaction schemes can open alternative reaction channels, and thus lead to new final products. Herein, we show for tetrahydrofuran (THF) cleavage on Si(001), the surface analogue of an SN 2 reaction, that excitation by electrons from the tip of a scanning tunneling microscope (STM) not only opens new reaction channels, but that different final products can be selectively addressed by the type of excitation: Above a threshold voltage of 2.5 V, direct excitation by electron transfer into the antibonding C-O orbital of the THF molecules induces ether cleavage of the datively bonded intermediate of THF on Si(001). Below the threshold, ether cleavage is induced by multiple excitation of vibrational modes. In both modes of excitation, additional final configurations were observed when compared to the thermally activated reaction. The branching ratios of the final configurations are different for the two different excitation mechanisms, which in turn can be controlled by the applied sample bias.

Functionalization and passivation of ultrathin alumina films of defined sub-nanometer thickness with self-assembled monolayers.

Instability of ultrathin surface oxides on alloys under environmental conditions can limit the opportunities for applications of these systems when the thickness control of the insulating oxide film is crucial for device performance. A procedure is developed to directly deposit self-assembled monolayers (SAM) from solvent onto substrates prepared under ultra-high vacuum conditions without exposure to air. As an example, rhenium photosensitizers functionalized with carboxyl linker groups are attached to ultrathin alumina grown on NiAl(1 1 0). The thickness change of the oxide layer during the SAM deposition is quantified by x-ray photoelectron spectroscopy and can be drastically reduced to one atomic layer. The SAM acts as a capping layer, stabilizing the oxide thin film under environmental conditions. Ultraviolet photoelectron spectroscopy elucidates the band alignment in the resulting heterostructure. The method for molecule attachment presented in this manuscript can be extended to a broad class of molecules vulnerable to pyrolysis upon evaporation and presents an elegant method for attaching molecular layers on solid substrates that are sensitive to air.

Centimeter-Sized Single-Orientation Monolayer Hexagonal Boron Nitride With or Without Nanovoids.

Large-area hexagonal boron nitride (h-BN) promises many new applications of two-dimensional materials, such as the protective packing of reactive surfaces or as membranes in liquids. However, scalable production beyond exfoliation from bulk single crystals remained a major challenge. Single-orientation monolayer h-BN nanomesh is grown on 4 in. wafer single crystalline rhodium films and transferred on arbitrary substrates such as SiO2, germanium, or transmission electron microscopy grids. The transfer process involves application of tetraoctylammonium bromide before electrochemical hydrogen delamination. The material performance is demonstrated with two applications. First, protective sealing of h-BN is shown by preserving germanium from oxidation in air at high temperatures. Second, the membrane functionality of the single h-BN layer is demonstrated in aqueous solutions. Here, we employ a growth substrate intrinsic preparation scheme to create regular 2 nm holes that serve as ion channels in liquids.

The impact of metalation on adsorption geometry, electronic level alignment and UV-stability of organic macrocycles on TiO2(110).

Metal complexes of the tetradentate bipyridine based macrocycle pyrphyrin (Pyr) have recently shown promise as water reduction catalysts in homogeneous photochemical water splitting reactions. In this study, the adsorption and metalation of pyrphyrin on stoichiometric TiO2(110) is investigated in ultrahigh vacuum by means of scanning tunneling microscopy, photoelectron spectroscopy, low-energy electron diffraction, and density functional theory. In a joint experimental and computational effort, the local adsorption geometry at low coverage, the long-range molecular ordering at higher coverage and the electronic structure have been determined for both the bare ligand and the cobalt-metalated Pyr molecule on TiO2. The energy level alignment of CoPyr/TiO2 supports electron injection into TiO2 upon photoexcitation of the CoPyr complex and thus renders it a potential sensitizer dye. Importantly, Co-incorporation is found to stabilize the Pyr molecule against photo-induced degradation, while the bare ligand is decomposed rapidly under continuous UV-irradiation. This interesting phenomenon is discussed in terms of additional de-excitation channels for electronically highly excited molecular states.

Site-Specific Reactivity of Ethylene at Distorted Dangling-Bond Configurations on Si(001).

Differences in adsorption and reaction energetics for ethylene on Si(001) are reported with respect to distorted dangling-bond configurations induced by hydrogen precoverage, as obtained by DFT calculations. This can help to understand the influence of surface defects and precoverage on the reactivity of organic molecules on semiconductor surfaces in general. The results show that the reactivity of surface dimers fully enclosed by hydrogen-covered atoms is essentially unchanged compared to the clean surface. This is confirmed by scanning tunneling microscopy measurements. On the contrary, adsorption sites with partially covered surface dimers show a drastic increase in reactivity. This is due to a lowering of the reaction barrier by more than 50 % relative to the clean surface, which is in line with previous experiments. Adsorption on dimers enclosed by molecule (ethylene)-covered surface atoms is reported to have a strongly decreased reactivity, as a result of destabilization of the intermediate state due to steric repulsion; this is quantified through periodic energy decomposition analysis. Furthermore, an approach for the calculation of Gibbs energies of adsorption based on statistical thermodynamics considerations is applied to the system. The results show that the loss in molecular entropy leads to a significant destabilization of adsorption states.

From porphyrins to pyrphyrins: adsorption study and metalation of a molecular catalyst on Au(111).

The molecular ligand pyrphyrin, a tetradentate bipyridine based macrocycle, represents an interesting but widely unexplored class of molecules. It resembles the well-known porphyrin, but consists of pyridyl subunits instead of pyrroles. Metal complexes based on pyrphyrin ligands have recently shown promise as water reduction catalysts in homogeneous photochemical water splitting reactions. In this study, the adsorption and metalation of pyrphyrin on a single crystalline Au(111) surface is investigated in an ultrahigh vacuum by means of scanning tunneling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy and density functional theory. Pyrphyrin coverages of approximately one monolayer and less are obtained by sublimation of the molecules on the substrate kept at room temperature. The molecules self-assemble in two distinct phases of long-range molecular ordering depending on the surface coverage. The deposition of cobalt metal and subsequent annealing lead to the formation of Co-ligated pyrphyrin molecules accompanied by a pronounced change of the molecular self-assembly. Electronic structure calculations taking the herringbone reconstruction of Au(111) into account show that the molecules are physisorbed, but preferred adsorption sites are identified where Co and the N atoms of the two terminal cyano groups are optimally coordinated to the surface Au atoms. An intermediate state of the metalation reaction is observed and the reaction steps for the Co metalation of pyrphyrin molecules on Au(111) are established in a joint experimental and computational effort.

Complex surface chemistry of an otherwise inert solvent molecule: tetrahydrofuran on Si(001).

The reaction of tetrahydrofuran (THF), an otherwise inert solvent molecule, on Si(001) was experimentally studied in ultra-high vacuum. Using scanning tunneling microscopy (STM) and photoelectron spectroscopy at variable temperature, we could both isolate a datively bound intermediate state of THF on Si(001), as well as the final configuration that bridges two dimer rows of the Si(001) surface after ether cleavage. The latter configuration implies splitting of the OC bond, which is typically kinetically suppressed. THF thus exhibits a hitherto unknown reactivity on Si(001).

Structural evolution of perfluoro-pentacene films on Ag(111): transition from 2D to 3D growth.

The structural evolution and thermal stability of perfluoro-pentacene (PF-PEN) thin films on Ag(111) have been studied by means of low-temperature scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). Well-defined monolayer films can be prepared by utilizing the different adsorption energy of mono- and multilayer films and selectively desorbing multilayers upon careful heating at 380 K, whereas at temperatures above 400 K, a dissociation occurs. In the first monolayer, the molecules adopt a planar adsorption geometry and form a well-ordered commensurate (6 × 3) superstructure where molecules are uniformly oriented with their long axis along the <110> azimuth. This molecular orientation is also maintained in the second layer, where molecules exhibit a staggered packing motif, whereas further deposition leads to the formation of isolated, tall islands. Moreover, on smooth silver surfaces with extended terraces, growth of PF-PEN onto beforehand prepared long-range ordered monolayer films at elevated temperature leads to needle-like islands that are uniformly aligned at substrate steps along <110> azimuth directions.