Toward Gigahertz Photodetection with Transition Metal Dichalcogenides.

ConspectusTransition metal dichalcogenides (TMDCs) exhibit favorable properties for optical communication in the gigahertz (GHz) regime, such as large mobilities, high extinction coefficients, cheap fabrication, and silicon compatibility. While impressive improvements in their sensitivity have been realized over the past decade, the bandwidths of these devices have been mostly limited to a few megahertz. We argue that this shortcoming originates in the relatively large RC constants of TMDC-based photodetectors, which suffer from high surface defect densities, inefficient charge carrier injection at the electrode/TMDC interface, and long charging times. However, we show in a series of papers that rather simple adjustments in the device architecture afford TMDC-based photodetectors with bandwidths of several hundreds of megahertz. We rationalize the success of these adjustments in terms of the specific physical-chemical properties of TMDCs, namely their anisotropic in-plane/out-of-plane carrier behavior, large optical absorption, and chalcogenide-dependent surface chemistry. Just one surprisingly simple yet effective pathway to fast TMDC photodetection is the reduction of the photoresistance by using light-focusing optics, which enables bandwidths of 0.23 GHz with an energy consumption of only 27 fJ/bit.By reflecting on the ultrafast intrinsic photoresponse times of a few picoseconds in TMDC heterostructures, we motivate the application of more demanding chemical strategies to exploit such ultrafast intrinsic properties for true GHz operation in real devices. A key aspect in this regard is the management of surface defects, which we discuss in terms of its dependence on the layer thickness, its tunability by molecular adlayers, and the prospects of replacing thermally evaporated metal contacts by laser-printed electrodes fabricated with inks of metalloid clusters. We highlight the benefits of combining TMDCs with graphene to heterostructures that exhibit the ultrafast photoresponse and large spectral range of Dirac materials with the low dark currents and high responsivities of semiconductors. We introduce the bulk photovoltaic effect in TMDC-based materials with broken inversion symmetry as well as a combination of TMDCs with plasmonic nanostructures as means for increasing the bandwidth and responsivity simultaneously. Finally, we describe the prospects of embedding TMDC photodetectors into optical cavities with the objective of tuning the lifetime of the photoexcited state and increasing the carrier mobility in the photoactive layer.The findings and concepts detailed in this Account demonstrate that GHz photodetection with TMDCs is feasible, and we hope that these bright prospects for their application as next-generation optoelectronic materials motivate more chemists and material scientists to actively pursue the development of the more complicated material combinations outlined here.

Nano-antennas with decoupled transparent leads for optoelectronic studies.

Performing electrical measurements on single plasmonic nanostructures presents a challenging task due to the limitations in contacting the structure without disturbing its optical properties. In this work, we show two ways to overcome this problem by fabricating bow-tie nano-antennas with indium tin oxide leads. Indium tin oxide is transparent in the visible range and electrically conducting, but non-conducting at optical frequencies. The structures are prepared by electron beam lithography. Further definition, such as introducing small gaps, is achieved by focused helium ion beam milling. Dark-field reflection spectroscopy characterization of the dimer antennas shows typical unperturbed plasmonic spectra with multiple resonance peaks from mode hybridization.

Sub-nanosecond Intrinsic Response Time of PbS Nanocrystal IR-Photodetectors.

Colloidal nanocrystals (NCs), especially lead sulfide NCs, are promising candidates for solution-processed next-generation photodetectors with high-speed operation frequencies. However, the intrinsic response time of PbS-NC photodetectors, which is the material-specific physical limit, is still elusive, as the reported response times are typically limited by the device geometry. Here, we use the two-pulse coincidence photoresponse technique to identify the intrinsic response time of 1,2-ethanedithiol-functionalized PbS-NC photodetectors after femtosecond-pulsed 1560 nm excitation. We obtain an intrinsic response time of ∼1 ns, indicating an intrinsic bandwidth of ∼0.55 GHz as the material-specific limit. Examination of the dependence on laser power, gating, bias, temperature, channel length, and environmental conditions suggest that Auger recombination, assisted by NC-surface defects, is the dominant mechanism. Accordingly, the intrinsic response time might further be tuned by specifically controlling the ligand coverage and trap states. Thus, PbS-NC photodetectors are feasible for gigahertz optical communication in the third telecommunication window.

Probing Bias-Induced Electron Density Shifts in Metal-Molecule Interfaces via Tip-Enhanced Raman Scattering.

Surface charging effects at metal-molecule interfaces, for example, charge transfer, charge transport, charge injection, and so on, have a strong impact on the performance of organic electronics. Only having molecules bound or adsorbed on different metals results in a doping-like behavior at the interface by the different work functions of the metals and creates hybrid surface states, which strongly affect the efficiencies. With the ongoing downsizing and thinning of the organic components, the impact of the interface will even further increase. However, most of the investigations only monitor the interface without the additional charging effects from applying a voltage to the interface. In this work we present a spectroscopic approach based on tip-enhanced Raman spectroscopy (TERS) to study metal-molecule interfaces with an applied voltage simulating the electric field strength in real devices. We monitor how an intrinsic inductive effect of partial functional groups in molecules can shift the molecular electron density (ED) distribution when a bias voltage is applied. Therefore, we choose two molecules as model systems, which are similar in size and binding condition to a smooth gold surface, but with different electronic structure. By placing the tip 1 nm over the molecular surface at a fixed position and changing the applied bias voltage, we record electric-field-dependent tip-enhanced Raman spectra. Specific vibrational bands exhibit voltage-dependent intensity changes related to the shift of the local ED inside the molecules. We believe this experiment is valuable to gain deeper insights into charged metal-molecule interfaces.

Light-Controlled Near-Field Energy Transfer in Plasmonic Metasurface Coupled MoS2 Monolayer.

The energy transfer from plasmonic nanostructures to semiconductors has been extensively studied to enhance light-harvesting and tailor light-matter interactions. In this study, the efficient energy transfer from an Au metasurface to monolayered MoS2 within a near-field coupling regime is reported. The metasurface is designed and fabricated to demonstrate strong photoluminescence (PL) and cathodoluminescence (CL) emission spectra. In the coupled heterostructure of MoS2 with a metasurface, both the Raman shift and absorption spectral intensities of monolayered MoS2 are affected. The spectral profile and PL peak position can be tailored owing to the energy transfer between plasmonic nanostructures and semiconductors. This is confirmed by ultrafast lifetime measurement. A theoretical model of two coupled oscillators is proposed, where the expanded general solutions (EGS) of such a model result in a series of eigenvalues that correspond to the renormalization of energy levels in modulated MoS2. The model can predict the peak shift up to tens of nanometers in hybrid structures and hence provides an alternative method to describe energy transfer between metallic structures and two-dimensional (2D) semiconductors. A viable approach for studying light-matter interactions in 2D semiconductors via near-field energy transfer is presented, which may stimulate the applications of functional nanophotonic devices.

Nanoscale plasmonic phase sensor.

Using the localized surface plasmon resonance (LSPR) of gold nanoparticles for sensing applications has attracted considerable interest, since it can be very sensitive, even down to a single molecule, and selective for a specific analyte molecule with a suitable surface modification. LSPR sensing is usually based on the wavelength shift of the LSPR or a Fano resonance. Here, we present a new experimental approach based on the phase of the light scattered by a single gold nanoparticle by equipping a confocal microscope with an additional interferometer arm similar to a Michelson interferometer. The detected phase depends on the shape of the nanoparticle and the refractive index of the surrounding medium and can even be detected for off-resonant excitation. This can be used as a new and sensitive detection method in LSPR sensing, allowing the detection of changes to the local refractive index or the binding of molecules to the nanoparticle surface.

Opportunities and challenges for electrochemistry in studying the electronic structure of nanocrystals.

We review the state-of-the-art of determining the electronic structure of nanocrystals in thin films by electrochemistry. Our core conclusion, the necessity of combining electrochemical with spectroscopic techniques, is illustrated with the holistic analysis of thin films of CdSe nanocrystals cross-linked with electroactive metal ß-tetraaminophthalocyanines by differential pulse voltammetry, optical spectroscopy and potential modulated absorption spectroscopy. We show that the same nanocrystals cross-linked with phthalocyanines of different metal centers exhibit rather similar electrochemical signatures, but behave distinctly different in spectroelectrochemical investigations. We argue that in one case, namely CdSe nanocrystals cross-linked with Co ß-tetraaminophthalocyanine, we find supporting evidence for the hybridization of energy levels at the organic/inorganic interface. This work suggests that spectroelectrochemistry is capable of revealing the electronic structure of complex nanomaterials, such as semiconductor nanocrystals functionalized with organic pi-systems.

Simultaneous positive and negative optical patterning with dye-sensitized CdSe quantum dots.

CdSe quantum dots are functionalized with the organic dye iron ß-tetraaminophthalocyanine to reward a solution-processable hybrid material with two individually addressable optical resonances. We exploit this dual functionality during optical write/optical read patterning experiments and show that it is possible to simultaneously write complex optical patterns with positive and negative fluorescence contrast. This is enabled by a fluorescence enhancement under near-resonant excitation of the quantum dots in combination with a fluorescence bleaching during excitation of the singlet transition of the phthalocyanine. The presence of the organic dye not only enables negative optical patterning but also enhances the contrast during positive patterning. Furthermore, the patterning result is strongly dependent on the excitation wavelength during readout. Our results highlight the new possibilities that arise from combining inorganic quantum dots and organic π-systems into hybrid nanocomposites.

Correlated, Dual-Beam Optical Gating in Coupled Organic-Inorganic Nanostructures.

An optical switch with two distinct resonances is formed by combining PbS nanocrystals and the conductive polymer poly[sodium 2-(2-ethynyl-4-methoxyphenoxy)acetate] (PAE) into a hybrid thin film. Infrared excitation of the nanocrystals invokes charge transfer and consecutive polaron formation in the PAE, which activates the switch for excited-state absorption at visible frequencies. The optical modulation of the photocurrent response of the switch exhibits highly wavelength-selective ON/OFF ratios. Transient absorption spectroscopy shows that the polaron formation is correlated with the excited state of the nanocrystals, opening up new perspectives for photonic data processing. Such correlated activated absorption can be exploited to enhance the sensitivity for one optical signal by a second light source of different frequency as part of an optical amplifier or a device with AND logic.

Nonlinear optical imaging of single plasmonic nanoparticles with 30 nm resolution.

We show that background free nonlinear optical imaging of Au nanostructures with a resolution down to 30 nm can be achieved. To attain such performance, an ultrafast laser source (110 fs pulse duration) has been integrated into a parabolic mirror assisted scanning near-field optical microscope. Combining nonlinear hyperspectral imaging and the simultaneously obtained topography, the setup allows one to directly correlate/assign locations with nonlinear signals originating from either second harmonic generation or two-photon excitation processes. The contrast mechanisms of the far-field background free nonlinear optical image are discussed based on the different tip-sample coupling schemes and the selective excitation of the plasmonic modes.

Inducing magnetism in pure organic molecules by single magnetic atom doping.

We report on in situ chemical reactions between an organic trimesic acid (TMA) ligand and a Co atom center. By varying the substrate temperature, we are able to explore the Co-TMA interactions and create novel magnetic complexes that preserve the chemical structure of the ligands. Using scanning tunneling microscopy and spectroscopy combined with density functional theory calculations, we elucidate the structure and the properties of the newly synthesized complex at atomic or molecular size level. Hybridization between the atomic orbitals of the Co and the π orbitals of the ligand results in a delocalized spin distribution onto the TMA. The here demonstrated possibility to conveniently magnetize such versatile molecules opens up new potential applications for TMAs in molecular spintronics.

Au nanotip as luminescent near-field probe.

We introduce a new optical near-field mapping method, namely utilizing the plasmon-mediated luminescence from the apex of a sharp gold nanotip. The tip acts as a quasi-point light source which does not suffer from bleaching and gives a spatial resolution of ≤25 nm. We demonstrate our method by imaging the near field of azimuthally and radially polarized plasmonic modes of nonluminescent aluminum oligomers.

Direct laser induced writing of high precision gold nanosphere SERS patterns.

The high sensitivity and molecular fingerprint capability of Surface-Enhanced Raman Spectroscopy (SERS) have lead to a wide variety of applications ranging from classical physics, chemistry over biology to medicine. Equally, there are numerous methods to fabricate samples owing to the desired properties and to create the localized surface plasmon resonances (LSPRS). However, for many applications the LSPRs must be specifically localized on micrometer sized areas and multiple steps of lithography are needed to achieve the desired substrates. Here we present a fast and reliable direct laser induced writing (DIW) method to produce SERS substrates with active areas of interest in any desired size and shape in the micrometer regime. Afterwards, the SERS substrates have been functionalized with phthalocyanines. The DIW fabricated samples realize sub-monolayer sensitivity and an almost uniform enhancement over the entire area, which make this production method suitable for many sensing applications.

Dual polarization-enabled ultrafast bulk photovoltaic response in van der Waals heterostructures.

The bulk photovoltaic effect (BPVE) originating from spontaneous charge polarizations can reach high conversion efficiency exceeding the Shockley-Queisser limit. Emerging van der Waals (vdW) heterostructures provide the ideal platform for BPVE due to interfacial interactions naturally breaking the crystal symmetries of the individual constituents and thus inducing charge polarizations. Here, we show an approach to obtain ultrafast BPVE by taking advantage of dual interfacial polarizations in vdW heterostructures. While the in-plane polarization gives rise to the BPVE in the overlayer, the charge carrier transfer assisted by the out-of-plane polarization further accelerates the interlayer electronic transport and enhances the BPVE. We illustrate the concept in MoS2/black phosphorus heterostructures, where the experimentally observed intrinsic BPVE response time achieves 26 ps, orders of magnitude faster than that of conventional non-centrosymmetric materials. Moreover, the heterostructure device possesses an extrinsic response time of approximately 2.2 ns and a bulk photovoltaic coefficient of 0.6 V-1, which is among the highest values for vdW BPV devices reported so far. Our study thus points to an effective way of designing ultrafast BPVE for high-speed photodetection.

Probing the electronic properties of trimesic acid nanoporous networks on Au(111).

Nowadays molecular nanoporous networks have numerous uses in surface nanopatterning applications and in studies of host-guest interactions. Trimesic acid (TMA), a benzene derivative with three carboxylic groups, is a marvelous building block for forming 2D H-bonded porous networks. Here, we report a low-temperature study of the nanoporous "chicken-wire" superstructure formed by TMA molecules adsorbed on a Au(111) surface. Distinct preferential orientations of the porous networks on Au(111) lead to the formation of peculiar TMA polymorphs that are stabilized only at the boundary between rotational molecular domains. Scanning tunneling microscopy (STM) and spectroscopy are used to investigate the electronic properties of both the molecular building blocks and the pores. Sub-molecular-resolution imaging and spatially resolved electronic spectroscopy reveal a remarkable change in the appearance of the molecules in the STM images at energies in the range of the lowest unoccupied molecular orbital, accompanied by highly extended molecular wave functions into the pores. The electronic structure of the pores reflects a weak confinement of surface electrons by the TMA network. Our experimental observations are corroborated by density-functional-theory-based calculations of the nanoporous structure adsorbed on Au(111).

Nanometer Sized Direct Laser-Induced Gold Printing for Precise 2D-Electronic Device Fabrication.

Flexible electronics manufacturing technologies are essential and highly favored for future integrated photonic and electronic devices. Direct laser induced writing (DIW) of metals has shown potential as a fast and highly variable method in adaptable electronics. However, most of the DIW procedures use silver structures, which tend to oxidize and are limited to the micrometer regime. Here, a DIW technique is introduced that not only enables electrical gold wiring of 2D van-der-Waals materials with sub-µm structures and 100 nm interspacing resolution but is also capable of fabricating photo switches and field effect transistors on various rigid and elastic materials. Light sensitive metalloid Au32 -nanoclusters serve as the ink that allows for low-power cw-laser exposure without further post-treatment. With a simple lift-off procedure, the unexposed ink can be removed. The technique realizes ultrafast, high resolution, and high precision production of integrated electronics and may pave the way for personalized circuits even printed on curved surfaces.

Quantitative measurement of the magnetic moment of individual magnetic nanoparticles by magnetic force microscopy.

The quantitative measurement of the magnetization of individual magnetic nanoparticles (MNPs) using magnetic force microscopy (MFM) is described. Quantitative measurement is realized by calibration of the MFM signal using an MNP reference sample with traceably determined magnetization. A resolution of the magnetic moment of the order of 10(-18) A m(2) under ambient conditions is demonstrated, which is presently limited by the tip's magnetic moment and the noise level of the instrument. The calibration scheme can be applied to practically any magnetic force microscope and tip, thus allowing a wide range of future applications, for example in nanomagnetism and biotechnology.

Aligned Stacking of Nanopatterned 2D Materials for High-Resolution 3D Device Fabrication.

Two-dimensional materials can be combined by placing individual layers on top of each other, so that they are bound only by their van der Waals interaction. The sequence of layers can be chosen arbitrarily, enabling an essentially atomic-level control of the material and thereby a wide choice of properties along one dimension. However, simultaneous control over the structure in the in-plane directions is so far still rather limited. Here, we combine spatially controlled modifications of 2D materials, using focused electron irradiation or electron beam induced etching, with the layer-by-layer assembly of van der Waals heterostructures. The presented assembly process makes it possible to structure each layer with an arbitrary pattern prior to the assembly into the heterostructure. Moreover, it enables a stacking of the layers with accurate lateral alignment, with an accuracy of currently 10 nm, under observation in an electron microscope. Together, this enables the fabrication of almost arbitrary 3D structures with highest spatial resolution.

Picosecond electrical response in graphene/MoTe2 heterojunction with high responsivity in the near infrared region.

Understanding the fundamental charge carrier dynamics is of great significance for photodetectors with both high speed and high responsivity. Devices based on two-dimensional (2D) transition metal dichalcogenides can exhibit picosecond photoresponse speed. However, 2D materials naturally have low absorption, and when increasing thickness to gain higher responsivity, the response time usually slows to nanoseconds, limiting their photodetection performance. Here, by taking time-resolved photocurrent measurements, we demonstrated that graphene/MoTe2 van der Waals heterojunctions realize a fast 10 ps photoresponse time owing to the reduced average photocurrent drift time in the heterojunction, which is fundamentally distinct from traditional Dirac semimetal photodetectors such as graphene or Cd3As2 and implies a photodetection bandwidth as wide as 100 GHz. Furthermore, we found that an additional charge carrier transport channel provided by graphene can effectively decrease the photocurrent recombination loss to the entire device, preserving a high responsivity in the near-infrared region. Our study provides a deeper understanding of the ultrafast electrical response in van der Waals heterojunctions and offers a promising approach for the realization of photodetectors with both high responsivity and ultrafast electrical response.

Spatially resolved fluorescence of caesium lead halide perovskite supercrystals reveals quasi-atomic behavior of nanocrystals.

We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffraction to reveal surface defects in supercrystals of self-assembled cesium lead halide perovskite nanocrystals and study their effect on the fluorescence properties. Upon comparison with density functional modeling, we show that a loss in structural coherence, an increasing atomic misalignment between adjacent nanocrystals, and growing compressive strain near the surface of the supercrystal are responsible for the observed fluorescence blueshift and decreased fluorescence lifetimes. Such surface defect-related optical properties extend the frequently assumed analogy between atoms and nanocrystals as so-called quasi-atoms. Our results emphasize the importance of minimizing strain during the self-assembly of perovskite nanocrystals into supercrystals for lighting application such as superfluorescent emitters.