Local Optoelectronic Properties Research Articles

Exploring the Local Optoelectronic Properties of ALD Grown TiO2 Photoelectrode-Coatings with Atomic Force Microscopy Methods

Photoelectrochemical cells (PECs) hold great promise as an environmentally friendly method of converting sunlight into valuable fuels. However, the semiconductor components of PEC photoelectrodes are susceptible to corrosion under photoelectrochemical conditions.In order to protect the surface of these semiconductors from degradation, a thin overlayer of Titanium dioxide (TiO2) is often applied by atomic layer deposition (ALD). In addition to their protecting function, the physicochemical properties of these thin films significantly affect the overall performance of the device. The local nano-scale optoelectronic characteristics, which considerably influence their macroscopic properties, have rarely been reported in the literature.AFM methods constitute a powerful tool to investigate photo-induced charge separation, charge transport and recombination on photoelectrode surfaces with nanometer resolution.In this context, we have employed Tunneling Conductive AFM (TUNA) and Kelvin Probe Force Microscopy (KPFM) in the dark and under illumination to investigate the surface of ALD-grown TiO2. The synthesis conditions determine the morphology, which has a strong impact on the local optoelectronic properties. TUNA measurements reveal higher photogenerated currents on the grain and facet boundaries, showing that these regions can be favorable conduction pathways. Based on KPFM experiments we can quantify the more efficient photo-induced charge separation on the crystalline TiO2 inclusions, which generate a significant surface photopotential (~450 mV) upon UV-illumination. Categorization of different regions of surface topography based on morphology allows us to analyze the evolution of surface potential over time and extract localized time constants for carrier dynamic processes on the films. This spatially resolved information provides valuable insight to guide the development of macroscopically stable and efficient photoelectrodes through the engineering of microstructure at the nano scale.

Super-Resolution Exciton Imaging of Nanobubbles in 2D Semiconductors with Near-Field Nanophotoluminescence Microscopy.

Two-dimensional (2D) semiconductors, such as transition metal dichalcogenides, have emerged as important candidate materials for next-generation chip-scale optoelectronic devices with the development of large-scale production techniques, such as chemical vapor deposition (CVD). However, 2D materials need to be transferred to other target substrates after growth, during which various micro- and nanoscale defects, such as nanobubbles, are inevitably generated. These nanodefects not only influence the uniformity of 2D semiconductors but also may significantly alter the local optoelectronic properties of the composed devices. Hence, super-resolution discrimination and characterization of nanodefects are highly demanded. Here, we report a near-field nanophotoluminescence (nano-PL) microscope that can quickly screen nanobubbles and investigate their impact on local excitonic properties of 2D semiconductors by directly visualize the PL emission distribution with a very high spatial resolution of ∼10 nm, far below the optical diffraction limit, and a high speed of 10 ms/point under ambient conditions. By using nano-PL microscopy to map the exciton and trion emission intensity distributions in transferred CVD-grown monolayer tungsten disulfide (1L-WS2) flakes, it is found that the PL intensity decreases by 13.4% as the height of the nanobubble increases by every nanometer, which is mainly caused by the suppression of trion emission due to the strong doping effect from the substrate. In addition to the nanobubbles, other types of nanodefects, such as cracks, stacks, and grain boundaries, can also be characterized. The nano-PL method is proven to be a powerful tool for the nondestructive quality inspection of nanodefects as well as the super-resolution exploration of local optoelectronic properties of 2D materials.

Ultrafast Electronic Relaxation Dynamics of Atomically Thin MoS2 Is Accelerated by Wrinkling.

Strain engineering is an attractive approach for tuning the local optoelectronic properties of transition metal dichalcogenides (TMDs). While strain has been shown to affect the nanosecond carrier recombination dynamics of TMDs, its influence on the sub-picosecond electronic relaxation dynamics is still unexplored. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and nonadiabatic ab initio molecular dynamics (NAMD) to investigate the ultrafast dynamics of wrinkled multilayer (ML) MoS2 comprising 17 layers. Following 2.41 eV photoexcitation, electronic relaxation at the Γ valley occurs with a time constant of 97 ± 2 fs for wrinkled ML-MoS2 and 120 ± 2 fs for flat ML-MoS2. NAMD shows that wrinkling permits larger amplitude motions of MoS2 layers, relaxes electron-phonon coupling selection rules, perturbs chemical bonding, and increases the electronic density of states. As a result, the nonadiabatic coupling grows and electronic relaxation becomes faster compared to flat ML-MoS2. Our study suggests that the sub-picosecond electronic relaxation dynamics of TMDs is amenable to strain engineering and that applications which require long-lived hot carriers, such as hot-electron-driven light harvesting and photocatalysis, should employ wrinkle-free TMDs.

Direct Visualization of Subnanometer Variations in the Excitonic Spectra of 2D/3D Semiconductor/Metal Heterostructures

The integration of metallic contacts with two-dimensional (2D) semiconductors is routinely required for the fabrication of nanoscale devices. However, nanometer-scale variations in the 2D/metal interface can drastically alter the local optoelectronic properties. Here, we map local excitonic changes of the 2D semiconductor MoS2 in contact with Au. We utilize a suspended and epitaxially grown 2D/metal platform that allows correlated electron energy-loss spectroscopy (EELS) and angle resolved photoelectron spectroscopy (nanoARPES) mapping. Spatial localization of MoS2 excitons uncovers an additional EELS peak related to the MoS2/Au interface. NanoARPES measurements indicate that Au-S hybridization decreases substantially with distance from the 2D/metal interface, suggesting that the observed EELS peak arises due to dielectric screening of the excitonic Coulomb interaction. Our results suggest that increasing the van der Waals distance could optimize excitonic spectra of mixed-dimensional 2D/3D interfaces and highlight opportunities for Coulomb engineering of exciton energies by the local dielectric environment or moiré engineering.

Scanning Tunneling Microscopy-Induced Light Emission and I(V) Study of Optical Near-Field Properties of Single Plasmonic Nanoantennas.

Electrically driven plasmonic nanoantennas can be integrated as a local source of the optical signal of advanced photonic schemes for on-chip data processing. The inelastic electron tunneling provides the photon generation or launch of surface plasmon waves. This process can be enhanced by the local density of optical states of nanoantennas. In this paper, we used scanning tunnel microscopy-induced light emission to probe the local optoelectronic properties of single gold nanodiscs. The electromagnetic field distribution in the vicinity of plasmonic structures was investigated with high spatial resolution. The obtained photon maps reveal the nonuniform distribution of electromagnetic near-fields, which is consistent with nanoantenna optical modes. Also, the analysis of derived I(V) curves showed a direct correlation between the nanoantenna optical states and the appearance of features on current-voltage characteristics.

Support vector machine for EELS oxidation state determination

Electron Energy-Loss Spectroscopy (EELS) is a powerful and versatile spectroscopic technique used to study the composition and local optoelectronic properties of nanometric materials. Currently, this technique is generating large amounts of spectra per experiment, producing a huge quantity of data to analyse. Several strategies can be applied in order to classify these data to map physical properties at the nanoscale. In the present study, the Support Vector Machine (SVM) algorithm is applied to EELS, and its effectiveness identifying EEL spectra is assessed. Our results evidence the capacity of SVM to determine the oxidation state of iron and manganese in iron and manganese oxides, based on the ELNES of the white lines of the transition metal. The SVM algorithm is first trained with given datasets and then the resulting models are tested through noisy test data sets. We demonstrate that SVM exhibits a very good performance classifying these EEL spectra, despite the usual level of noise and instrumental energy shifts.

Polarization-Dependent Light Emission and Charge Creation in MoS2 Monolayers on Plasmonic Au Nanogratings.

We fabricated plasmonic hybrid nanostructures consisting of MoS2 monolayer flakes and Au nanogratings with a period of 500 nm. The angle-resolved reflectance and photoluminescence spectra of the hybrid nanostructures clearly indicated a coupling between surface plasmon polaritons (SPPs) and incoming photons. The surface photovoltage (SPV) maps could visualize the spatial distribution of net charges while shining light on the sample. Considerable polarization and wavelength dependence of the SPV signals suggested that the SPP mode enhanced the light-matter interaction and resulting exciton generation in the MoS2 monolayer. From the photoluminescence spectra and the morphology of the suspended MoS2 region, it could be noted that light irradiation did not much raise the temperature of the MoS2 monolayers on the nanogratings. Nanoscopic SPV and surface topography measurements could reveal the local optoelectronic and mechanical properties of MoS2 monolayers. This work provided us insights into the proposal of a high-performance MoS2/metal optoelectronic devices, based on the understanding of the SPP-photon and SPP-exciton coupling.

Strong modulation of optical properties in rippled 2D GaSe via strain engineering

Few-layer GaSe is one of the latest additions to the family of two-dimensional semiconducting crystals whose properties under strain are still relatively unexplored. Here, we study rippled nanosheets that exhibit a periodic compressive and tensile strain of up to 5%. The strain profile modifies the local optoelectronic properties of the alternating compressive and tensile regions, which translates into a remarkable shift of the optical absorption band-edge of up to 1.2 eV between crests and valleys. Our experimental observations are supported by theoretical results from density functional theory calculations performed for monolayers and multilayers (up to seven layers) under tensile and compressive strain. This large band gap tunability can be explained through a combined analysis of the elastic response of Ga atoms to strain and the symmetry of the wave functions.

Spatial Inhomogeneity Analysis of Cesium-Rich Wrinkles in Triple-Cation Perovskite

Chemical composition engineering in metal halide perovskite leads to enhanced stability and better transport properties, opening the gate to their integration in competitive photovoltaic tandem devices or LEDs. However, triple-cation perovskites show morphological, chemical, optical, and optoelectronic heterogeneities. In this study, we focus on micrometric spatial inhomogeneities where we observe wrinkle formation in the fabrication process linked to the cesium addition. Electron-dispersive spectroscopy and photoluminescence (PL) hyperspectral imaging designate these morphological features as Cs-rich and N-poor, which underlines the role of long-range chemical migration in the perovskite formation. We also study charge-carrier diffusion and recombination using time-resolved PL imaging under wide-field illumination, which we correlate with 3D drift diffusion modeling. We underline the impact of wrinkles on local optoelectronic properties such as the bandgap opening due to Cs enrichment and the longer charge carrier lifetime due to lower trap density.

Local Optoelectronic Characterization of Solvent-Annealed, Lead-Free, Bismuth-Based Perovskite Films.

Traditional organolead-halide perovskite-based devices have shown rapid improvements in their power conversion efficiency in less than a decade, yet challenges remain for improving stability and film uniformity, as well as the elimination of lead to address toxicity issues. We fabricated lead-free methylammonium bismuth iodide (MBI) perovskite films and studied the effect of solvent annealing with dimethylformamide (DMF) on both (1) the crystallinity and structure of the films with X-ray diffraction and scanning electron microscopy and (2) the local optoelectronic properties of the films as measured via (photo)conductive atomic force microscopy. We found that solvent annealing leads to improved crystallinity and increased grain size in the MBI films as compared to the thermally annealed films. Furthermore, solvent-annealed MBI films show significantly increased electrical conductivity in the out-of-plane direction. Photoconductivity in both solvent-annealed and thermally annealed MBI films was increased in the grain interiors versus the grain boundaries. It was observed that DMF-induced solvent annealing impacts charge transport through the film, which can be a unique design parameter for optimizing local optoelectronic properties. By studying how solvent annealing affects the MBI film structure and changes the ways in which charges are transported through the film, we have developed a better understanding of how local optoelectronic properties are affected by DMF annealing.

Fabrication of optical fibre pencil shaped tips for nano objective imaging using home made tapering technique

A method is developed to produce chemically etched optical fibre tips for small and nano objective imaging with valuable properties such as large transmission and no lateral light leaks. Tips are coated with a dielectric material like Al, Cr, etc., that is refraction-index approximately matched to silica. First small objective imaging of latex nanospheres with the smallest optical aperture tips confirms their ability to reach subwavelength optical resolution. In addition, thanks to their high transmission and collection efficiencies as well as their natural achromatism. Probing optical properties of materials and optical characterisation of molecular defects at the nanometre scale has been inaccessible until recently owing to the diffraction limit of light. With the invention of nanometric fibre optic tip by the help of chemical etching and tapering technique, resolution at the 50 ± 100 nm level using visible or near infrared light is now practical. The unique capability of fibre optic probe to simultaneously measure surface topography and local optoelectronic properties, thereby eliminating the need to perform cross correlation analysis on results obtained using different techniques, is particularly useful in this area. The noticeable improvement of tapering technique was observed here.

Electrochemical and atomic force microscopy investigations of the effect of CdS on the local electrical properties of CH3NH3PbI3:CdS perovskite solar cells

The effect of the incorporated CdS on the local optoelectronic properties of CH3NH3PbI3:CdS bulk heterojunction (BHJ) perovskite solar cells (PSCs) are studied using Kelvin probe force microscopy (KPFM), conductive atomic force microscopy (c-AFM) and electrochemical impedance spectroscopy (EIS).

Influence of N,N-Dimethylformamide Annealing on the Local Electrical Properties of Organometal Halide Perovskite Solar Cells: an Atomic Force Microscopy Investigation

Organometal halide perovskites have been recognized as a new class of materials for photovoltaic application. Solvent annealing introduced during crystallization of bulk or thin-film materials can improve the performance of perovskite solar cells. Herein, we present Kelvin probe force microscopy and conductive atomic force microscopy (c-AFM) measurements to investigate the local optoelectronic properties of a perovskite film after N,N-dimethylformamide (DMF) annealing. AFM results show that DMF annealing induces recrystallization, yielding a large-size polycrystalline perovskite film. Uniform and higher photocurrent is distributed on the film. However, lower surface potential enhancement and photocurrent are observed at grain boundaries (GBs), illustrating that GBs acting as recombination sites are detrimental to photocurrent transport and collection. Our observation provides a nanoscale understanding of the device performance improvement after DMF annealing.

On describing the optoelectronic characteristics of poly(benzodithiophene-co-quinoxaline)-fullerene complexes: the influence of optimally tuned density functionals.

Here, we investigate the effects of both tuning the range-separation parameter of long-range corrected (LRC) density functionals and including dispersion corrections on describing the local optoelectronic properties of polymer-fullerene interfaces that are critical to the performance of polymer solar cells (PSCs). Focusing on recently studied (Chen, et al., Chem. Mater., 2012, 24, 4766-4772) PSC active layers derived from poly(benzodithiophene-co-quinoxaline) and substituted fullerene PC71BM, we compare the performance of global hybrid functionals (B3LYP and B3LYP-D) alongside two LRC functionals (ωB97X and ωB97X-D) and their optimally tuned (OT) analogs (OT-ωB97X and OT-ωB97X-D). Our results confirm that OT-LRC functionals generally improve the description of the optical properties of the individual materials with respect to experiment. For electron-donor (eD)-electron-acceptor (eA) complexes used to describe the local optoelectronic properties of the material interface, PC71BM is found to preferentially settle near the quinoxaline acceptor units on the copolymer backbone, regardless of the functional, though dispersion corrections have a strong influence on the intermolecular distances and, in turn, the nature of the excited states. All functionals yield very similar descriptions of the transition maxima for the complexes, i.e. predominant local excitations on the copolymer. Importantly, tuning the range-separation parameter of the LRC functional is shown to have a profound effect, as the OT functionals allow for the description of the charge transfer states of the eD-eA complexes, while the non-tuned LRC functionals predict only local intramolecular excitations. These results hold considerable importance for deriving the appropriate physical understanding of the interfacial structure-property-function relationships of PSCs.

Ultrathin single-walled carbon nanotube network framed graphene hybrids.

Graphene and single-walled carbon nanotubes (SWNTs) have shown superior potential in electronics and optoelectronics because of their excellent thermal, mechanical, electronic, and optical properties. Here, a simple method is developed to synthesize ultrathin SWNT-graphene films through chemical vapor deposition. These novel two-dimensional hybrids show enhanced mechanical strength that allows them to float on water without polymer supporting layers. Characterizations by Raman spectroscopy and transmission electron microscopy indicate that SWNTs can interlace as a concrete backbone for the subsequent growth of monolayer graphene. Optical and electrical transport measurements further show that SWNT-graphene hybrids inherit high optical transparency and superior electrical conductivity from monolayer graphene. We also explore the local optoelectronic properties of SWNT-graphene hybrids through spatially resolved photocurrent microscopy and find that the interactions between SWNTs and graphene can induce a strong photocurrent response in the areas where SWNTs link different graphene domains together. These fundamental studies may open a door for engineering optoelectronic properties of SWNT-graphene hybrids by controlling the morphologies of the SWNT frames.

Photocurrent contribution from inter-segmental mixing in donor–acceptor-type polymer solar cells: A multiscale simulation study

Polymer solar cells possess a promising perspective for generating renewable energy at affordable costs, provided their performance and durability can be improved considerably. To this end, several experimental and theoretical techniques have been devised recently, establishing a direct link between local morphology, local opto-electronic properties and device performance. However, their reliability is still unclear to this day. Here, we demonstrate by using a recently developed particle-based multiscale solar cell approach and comparing its results with the ones of a field-based solar cell algorithm that inter-mixing of the electron-donor(D)- and -acceptor(A)-type of segments in a lamellar-like poly(9,9’-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine)-poly(9,9'-dioctylfluorene-co-benzothiadiazole) (PFB-F8BT) blend causes that the major part of the charge generation and charge transport takes place inside the nanophases of the nanostructured polymer solar cells in agreement with recent experimental measurements and not, as commonly believed, at the visible domain boundaries of the DA interface. Moreover, we show that the contribution of the exciton dissociation efficiency to the internal quantum efficiency, due to inter-monomeric mixing, is significant and cannot be neglected in simulation studies at the nanoscale. Finally, we demonstrate that keto-defects on the fluorene moiety of the F8BT phase, induced by photo-oxidation, causes a simultaneous increase of the intra-chain contribution and decrease of the inter-chain contribution to the electronic current density, whereas in the reduced form the difference between both contributions is significantly smaller. This antagonistic effect leads to keto-induced electron trapping, resulting in a deteriorated electronic transport efficiency in devices with a photo-oxidized F8BT phase.

Photoconductive NSOM for mapping optoelectronic phases in nanostructures

AbstractThe advent of optically functional materials with low-intensive processing methods is accompanied by a growing need for high resolution imaging to probe the inherent inhomogeneities in the underlying microstructure. Atomic force microscopy based techniques are typically utilized for imaging the surface of organic thin films, quantum dots and other nanomaterials with ultrahigh resolution. Several modes like conductive, Kelvin, electrostatic amongst others have been particularly successful in imaging the local current, potential and charge distribution of variety of systems. However, the functionality of photoconduction in these materials cannot be directly imaged by these modes alone. There is a requirement for a local excitation source or collection arrangement that is compatible with scanning microscopy techniques followed by a current monitoring mechanism. Near-field scanning optical microscopy (NSOM) possesses all the advantages of scanning microscopy and is capable of local excitation that overcomes the diffraction limit faced by conventional optical microscopes. Additionally, NSOM can be carried out on actual photoconductive two terminal and three terminal device structures to image local optoelectronic properties. In this review, we present the various geometries that have been demonstrated to perform photoconductive NSOM (p-NSOM). We highlight a representative set of important results and discuss the implications of photocurrent imaging in macroscopic device performance.

Heterogeneity in Polymer Solar Cells: Local Morphology and Performance in Organic Photovoltaics Studied with Scanning Probe Microscopy

The use of organic photovoltaics (OPVs) could reduce production costs for solar cells because these materials are solution processable and can be manufactured by roll-to-roll printing. The nanoscale texture, or film morphology, of the donor/acceptor blends used in most OPVs is a critical variable that can dominate both the performance of new materials being optimized in the lab and efforts to move from laboratory-scale to factory-scale production. Although efficiencies of organic solar cells have improved significantly in recent years, progress in morphology optimization still occurs largely by trial and error, in part because much of our basic understanding of how nanoscale morphology affects the optoelectronic properties of these heterogeneous organic semiconductor films has to be inferred indirectly from macroscopic measurements. In this Account, we review the importance of nanoscale morphology in organic semiconductors and the use of electrical scanning probe microscopy techniques to directly probe the local optoelectronic properties of OPV devices. We have observed local heterogeneity of electronic properties and performance in a wide range of systems, including model polymer-fullerene blends such as poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM), newer polyfluorene copolymer-PCBM blends, and even all polymer donor-acceptor blends. The observed heterogeneity in local photocurrent poses important questions, chiefly what information is contained and what is lost when using average values obtained from conventional measurements on macroscopic devices and bulk samples? We show that in many cases OPVs are best thought of as a collection of nanoscopic photodiodes connected in parallel, each with their own morphological and therefore electronic and optical properties. This local heterogeneity forces us to carefully consider the adequacy of describing OPVs solely by "average" properties such as the bulk carrier mobility. Characterizing this local heterogeneity in the morphology of an OPV and the consequent variations in local performance is vital to understanding OPV operation.

Time-resolved electrostatic force microscopy of polymer solar cells

Blends of conjugated polymers with fullerenes, polymers, or nanocrystals make promising materials for low-cost photovoltaic applications. Different processing conditions affect the efficiencies of these solar cells by creating a variety of nanostructured morphologies, however, the relationship between film structure and device efficiency is not fully understood. We introduce time-resolved electrostatic force microscopy (EFM) as a means to measure photoexcited charge in polymer films with a resolution of 100 nm and 100 micros. These EFM measurements correlate well with the external quantum efficiencies measured for a series of polymer photodiodes, providing a direct link between local morphology, local optoelectronic properties and device performance. The data show that the domain centres account for the majority of the photoinduced charge collected in polyfluorene blend devices. These results underscore the importance of controlling not only the length scale of phase separation, but also the composition of the domains when optimizing nanostructured solar cells.

Near-field scanning optical microscopy studies of electronic and photonic materials and devices

Probing optical properties of materials and optical characterization of crystallographic defects at the nanometer scale have been inaccessible until recently due to the diffraction limit of light. With the invention of near-field scanning optical microscopy (NSOM), resolution at the 50–100 nm level using visible or near infrared light is now practical. In addition to describing the NSOM technique, this review focuses on the application of NSOM to the characterization of electronic and photonic materials and devices, with particular emphasis on defects. The unique capability of NSOM to simultaneously measure surface topography and local optoelectronic properties, thereby eliminating the need to perform cross correlation analysis on results obtained using different techniques, is particularly useful in this area. Several examples are discussed. By performing near-field photocurrent (NPC) measurements, NSOM is used to probe electrical activities associated with individual threading dislocations and dislocation networks in strain relaxed, compositionally graded GeSi films. The non-destructive nature of NSOM helps elucidate how microstructural defects in the SrTiO3 bicrystal substrates affect YBa2Cu3O7 film growth and GBJJ performance. Characterization of III–V and II–VI semiconductors, guantum dots grown by strain epitaxy, laser diodes, waveguides, and photonic crystals is also included. The advantages and disadvantages of NSOM in each application will be outlined. Throughout the review, emphasis is placed on how NSOM complements existing materials characterization techniques, as well as how quantitative results can be obtained from NSOM measurements.