This PDF file contains the front matter associated with SPIE Proceedings Volume 10724, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

The features in linear absorption spectra can be exquisitely sensitive to the electronic coupling between organic molecules in a molecular aggregate. The spectral signatures of molecular aggregation are the result of electronic coupling, which is determined by the physical arrangement of the molecules. In this work, the absorbance of pseudoisocyanine (PIC) is measured in situ after solution drop casting to reveal a distinct intermediate stage during the aggregation process. A possible composition and structure for the molecular aggregates during this stage is inferred by using a Holstein-like Hamiltonian to calculate an absorption spectrum with spectral features that match those of the measured spectrum. More than one type of aggregate is required to compute a spectrum that agrees with the measured spectrum within this model. In this case, the spectrum can be fit with a trimer and an aggregate with 9 molecules with electronic coupling values of +600 cm^-1 and -600 cm^-1, respectively. We report a procedure to compute spectra that agree with measured spectra and limits the number of iteratively fit parameters. This strategy will enable the interpretation of in situ absorption data for other conjugated molecules during molecular aggregation and provide insight into the evolving composition of aggregates during the process of film formation.

Continuous increase in the device performance of lead halide perovskite-based solar cells is strongly related to better understanding of the optoelectronic processes occurring in the perovskite layer and its interfaces. There are many of these processes that are critical to device performance, but are not yet fully understood, which include charge carrier accumulation and recombination, trapping of electrons and holes, and ionic movement. Here we report our recent results of methylammonium lead iodide (MAPI)-based photovoltaic devices identifying the origins of different open circuit voltages and their potential loss mechanisms in conventional and inverted device structures. We have investigated in detail the energetics and the illumination generated surface photovoltage (SPV) and its transient behaviour at the perovskite layer and its heterointerfaces with various charge extracting interlayers. A MAPI layer with different thicknesses was deposited on top of the various underlayers including ITO, n-type TiO2, p-type PEDOT:PSS and many oxides and organic semiconductors. We found that the work function of MAPI is strongly influenced by the underlayer showing generally p-type semiconductor character. The results of thickness dependent SPV measurements indicate that there is an increase in the hole concentration at both PEDOT:PSS/MAPI and TiO2/MAPI interfaces, which leads to an increased interfacial charge recombination. In this talk, I will discuss how these observations are related to different open circuit voltages and their loss in conventional and inverted devices. I will also discuss the temperature dependent transient SPV results, which is used to distinguish different processes governed by charge carrier generation, ion migration, and charge trapping – three processes taking place at three different timescales.

Solar cells based on organic-inorganic metal halide perovskites show efficiencies close to highly-optimized silicon solar cells. However, ion migration causes current-voltage hysteresis and long-term degradation, which impedes large-scale commercial applications. I will show that transient ion-drift measurements can be a powerful tool to study the activation energy, concentration, and diffusion coefficient of mobile ions, when measured within the correct frequency range, and with suitable delay times. In methylammonium lead triiodide (MAPbI3) perovskite, we identified three migrating ion species which we attribute to the migration of iodide (I-) and methylammonium (MA+). The estimated activation energies for the migration of mobile ions in the tetragonal phase are 0.37 eV for I- and 0.95 eV for MA+ near grain boundaries. The latter changes to 0.28 eV near the tetragonal-to-cubic phase-transition temperature. In the cubic phase, we find an additional activation energy of 0.43 eV which we attribute to the migration of MA+ ions in the bulk. We find that the concentration of mobile MA+ ions is significantly higher than the one of mobile I- ions, and that the diffusion coefficient of mobile MA+ ions is three to four orders of magnitude lower than the one for I- ions. From the associated timescale, we conclude that MA+ ions are mainly responsible for the observed current-voltage hysteresis in solar cells at typical operating temperatures. I will also present first sights into the influence of processing on the mobility of ions, a quantification which could lead to a better understanding of ion migration and its role in degradation of perovskite solar cells.

Organic and inorganic materials are more and more frequently combined in high-performance hybrid electronic and photonic devices. For such multilayered stacks, the identification of layers and interface defects by depth profile analysis is a challenging task, especially because of the possible ion beam induced modifications. This is particularly true for perovskite solar cells stacks that in a mesoscopic structure usually combine a metal electrode, a mesoscopic conductive oxide layer, an intrinsically hybrid light absorber, an organic hole extraction layer and a metal counter electrode. While depth profile analysis with X-ray photoelectron spectroscopy (XPS) was already applied to investigate these devices, the X-ray and ion beam induced modifications on such hybrid layers have not been previously investigated. In this work we compare the profiles obtained with monatomic Ar⁺ beam at different energies, with the ones obtained with argon ion clusters (Ar_n⁺) with different sizes (150<n<1000) and energies (up to 8 keV). A systematic study is performed on full mesoscopic perovskite (CH₃NH₃PbI₃) solar cells and on model hybrid samples ((FA_xCs_1-xPbI₃)0.85 (MAPbBr₃)_0.15)/TiO₂). The results show that for monatomic beams, the implantation of positively charged atoms induces the surface diffusion of free iodine species from the perovskite which modifies the I/Pb ratio. Moreover, lead atoms in the metallic state (Pb⁰ ) are found to accumulate at the bottom of the perovskite layer where the Pb⁰ /Pb_tot fraction reaches 50%. With argon clusters, the ion beam induced diffusion of iodine is reduced only when the etch rate is sufficiently high to ensure a profile duration comparable with low-energy Ar⁺. Convenient erosion rates are obtained only for n=300 and n=500 clusters at 8 keV, which have also the advantage of preserving the TiO₂ surface chemistry. However, with argon cluster ions, Pb⁰ particles in the perovskite are less efficiently sputtered which leads to the increase of the Pb⁰ /Pb_tot fraction (up to 75%) at the perovskite/TiO₂ interface. Finally, ion beam and X-ray induced artifacts on perovskite absorbers can be reasonably neglected for fast analysis conditions in which the exposure time is limited to few hours.

In recent years, lead-based perovskites have become the most promising photovoltaic material showing excellent photovoltaic performances. However, the lead-based perovskite encounters instability and toxicity issues caused by the generation of Pb²⁺, which is highly not beneficial to environment. In this study, we firstly reported the synthesis and characterization of a novel lead-free mixed chalcogen-halogen perovskite material, MABiI₂S (MBIS), and determined its physical and optical properties by various testing methods. The MBIS possesses a low bandgap of 1.52 eV and exhibits good absorptions across the visible light spectrum. It is also worth noting that the MBIS displayed absorption up to over 1000 nm.

Blue nanocrystal perovskite LEDs have traditionally lagged behind their red and green cousins. Here, we discuss the reasons for this lag and propose solutions to these problems, producing high efficiency blue perovskite LEDs. We demonstrate the NiOx, a transport material in one of the highest performing devices to date, reduces the performance of nanocrystals near to the interface. By replacing it with an alternative transport structure, we show that the nanocrystal emission is unperturbed. We then build full LEDs out of this transport structure, increasing the EQE from 0.03% to 0.50%, the highest for inorganic perovskite nanocrystals at this wavelength. We further show that the benefits of this transport structure relax as the energetics redshift, as our blue-green devices match those from literature. These results are a useful step forward towards commercially relevant perovskite LEDs.

Halide perovskites are currently of interest for a variety of optoelectronic applications. While, typical wet-chemical preparation techniques afford relatively rough polycrystalline layers, we have recently demonstrated that thermal imprint is a powerful post-deposition processing tool that affords extremely smooth perovskite thin-films with crystals that extend over tens of microns laterally.[1,2] A comparative study of optical, morphological and thermal properties (e.g. thermal conductivity) reveals some striking similarity of pressed MAPbX3 thin-films and their single crystalline analogues.[3] More recently, we successfully used thermal imprint also for entirely inorganic halide perovskite materials, such as CsPbBr3. While as-deposited CsPbBr3 layers are typically discontinuous and rough with a large number of pinholes, thermal imprint at relatively low temperature and pressure (150°C, 100 bar) will be shown to turn them into dense, smooth and pinhole-free thin films, which show substantially enhanced luminescence quantum yield and in contrast to pristine CsPbBr3 layers even enable room-temperature amplified spontaneous emission (ASE). Perovskite thin films patterned by thermal nanoimprint with photonic resonator structures will be shown to afford hybrid and entirely inorganic distributed feedback lasers, with ultra-low lasing thresholds.[2,4] [1] A. Mayer et al. J. Vac. Sci. & Techol. B 2017, 35, 06G803. [2] N. Pourdavoud et al. Adv. Mater. Technol. 2018, 3, 1700253. [3] R. Heiderhoff et al. J. Phys. Chem. C 2017, 121, 28306. [4] N. Pourdavoud et al. Adv. Mater. 2017, 29, 1605003.

We report the first observation of charge-transfer (CT) states at 2D metal halide perovskite/organic heterojunctions. The 2D perovskite (BAI)2(MAI)n-1(PbI2)n with various n values are used to form heterojunctions with various organic molecules (BA represents n-butylammonium and MA represents methylammonium). Charge-transfer features are found in the external quantum efficiency (EQE) versus wavelength curves only for the strong excitonic BA2PbI4 (n=1) case when forming a heterojunction with 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN). Additionally, the photocurrent output from the excitonic perovskite is enhanced in the presence of the CT feature, indicating the formation of a donor/acceptor interface for exciton dissociation. In contrast, CT features are not found at interfaces that are not dissociating, such as with tris-(8-hydroxyquinoline)aluminum (Alq3). Furthermore, as n increases, 2D perovskites are not sufficiently excitonic (i.e. the exciton binding energy reduces to less than or equal to kT) to show CT states, even with the strong electron acceptor HAT-CN. Observation of CT states at 2D perovskite/organic heterojunctions points to methods to exploit the excitonic nature of 2D perovskites to broader research areas such as donor-acceptor type solar cells, photodetectors, light emitting devices, and light-matter interactions.

Optical pump-terahertz probe spectroscopy is a powerful contact-free technique for probing the electronic properties of novel nanomaterials and their response to photoexcitation. This technique can measure charge carrier transport and dynamics with sub-picosecond temporal resolution. Electrical conductivity, charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities can be measured with high accuracy and with considerably higher throughput than achievable with traditional contact-based techniques. We describe how terahertz spectroscopy is revealing the fascinating properties and guiding the development of a number of promising semiconductor materials, with particular emphasis on III-V semiconductor nanowires and devices.

Anharmonic crystal structure dynamics have been observed in halide perovskites (APbX3, A = CH3NH3, Cs / X = I, Br) on picosecond timescales. Here, we report that the soft nature of the perovskite crystal lattice gives rise to dynamic fluctuations in the electronic properties of excited states. We use linear polarization selective transient absorption spectroscopy (LP-TA) to investigate how such crystal dynamics affect the electronic states occupied by photoexcited carriers in hybrid metal halide perovskite thin films (CH3NH3PbX3, X = I,Br) and nanocrystals (CsPbI3) at room temperature. This method is sensitive to the coupling between the optical polarization vector of the absorbed light and the transition dipole matrix (TDM) element of the electronic states, which allows us to probe optical anisotropies in the excited state population. Optical alignment upon linearly-polarized excitation occurs in a range of semiconductors, with a variety of underlying causes. In GaAs, the dependence of the optical TDM on the angular momentum of the electronic wave functions imprints a short-lived anisotropic carrier momentum distribution on the excited state population. This is lost through femtosecond carrier-carrier scattering. By contrast, optical alignment in molecular materials, with more localised excitonic states, may arise from an alignment of TDM with physical structure. Loss of polarization memory in this case arises from physical reorientation of the photoexcited molecule or diffusion of the excited state to regions with different dipole matrix orientation. We show that hybrid perovskites lie between these two extremes. Their soft structural nature allows dynamic symmetry breaking of the delocalised electronic states, preserving optical alignment over picoseconds, which is far beyond the timescale of momentum-scattering events. We also find that optical alignment is lost on the timescales of local structural reorientation rather than diffusion. We condiser this reorientation to occur in fluctuating polar nano-regions of the lead-halide lattice. We suggest that these electronically distinct regions represent the fundamental units of perovskite electronic structure, rather than crystal domains or individual nanocrystals. The electronically soft nature of halide perovskite crystals gives rise to behaviour not observed either in classical organic or inorganic semiconductors, which has far-reaching implications for the understanding and application of this important class of materials.

Interest in the hybrid organic-inorganic perovskite semiconductors has skyrocketed in recent years due to unprecedented high performance as solution-processable absorber layers in solar cells [1]. These materials also have potential for applications in semiconductor spintronics due to their large spin-orbit interaction. Theoretical studies predict a large Rashba spin splitting [2], and recent experiments have revealed a large photoinduced magnetization [3-5] and spin-dependent optical Stark effect [6], yet still relatively little is known about the spin-related properties of these materials. Here we report polarization-dependent pump probe studies of the 2D perovskite butylammonium methylammonium lead iodide. Our experiments indicate a strong influence of the Rashba spin splitting on the carrier kinetics in this system, consistent with our recent four-wave mixing studies of bulk CH3NH3PbI3. [1] https://www.nrel.gov/pv/assets/images/efficiency_chart.jpg. [2] M. Kepenekian and J. Even, J. Phys. Chem. Lett. 8, 3362 (2017). [3] D. Giovanni et al. Nano Lett. 15, 1553 (2015). [4] C. Zhang et al. Nat. Phys. 11, 427 (2015). [5] P. Odenthal et al. Nat. Phys. 13, 894 (2017). [6] D. Giovanni et al. Science Advances 2, e1600477 (2016).

My talk will address three related topics in organic solar cells. I will initially address the issue of whether interfacial electron / hole pairs (i.e.: charge transfer states) are bound and unbound in organic donor / acceptor blends. In particular I will present a range of kinetic and structural data indicating that the behavior of such states depend strongly upon interface structure. Interfaces within molecular mixed domains can yield relatively bound states which can undergo significant geminate recombination losses. In contrast, for the same donor / acceptor materials, interfaces between domains can yield efficient charge separation with minimal geminate losses. I will then go on to address the importance of geminate recombination losses in limiting photocurrent generation in polymer / non-fullerene acceptor solar cells, and the role of energy level tuning in minimizing such losses. Finally I will go to address the challenge of achieving efficient charge collection in organic solar cells with active layer thicknesses large enough for scalable device manufacture. In particular I will address the role of space charge layer formation caused by charge trapping in shallow tail states, and the requirement that efficient photocurrent generation in thick organic solar cells requires minimizing the density of these tail states.

Diketopyrrolopyrrole (DPP) is one of the most common building blocks for small molecules and conjugated polymers designed for organic electronic applications. Transient absorption spectroscopy (TAS) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy were used to examine bulk heterojunction blend films of a small diketopyrrolopyrrole-based molecule, TDPP, with the common fullerene derivatives PC60BM and PC70BM. Following pulsed laser excitation, the spectral signatures of a fullerene anion and a TDPP triplet state are observed on the picosecond timescale by TAS. The presence of these species imply the formation of a TDPP:PCBM charge transfer state that subsequently undergoes ultra-fast spin-mixing and geminate recombination to produce a TDPP triplet state. The overall photophysical mechanism is confirmed by TR-EPR spectroscopy, which unambiguously shows that the TDPP triplet is formed via spin-mixing in the TDPP:PCBM charge transfer state, rather than direct intersystem crossing from the excited singlet state. Furthermore, ultra-low band gap polymers INDT were investigated further using transient absorption spectroscopy (TAS) and pump-push photocurrent measurements. Different fullerenes were trialled to assess the effect on charge photogeneration. The LUMO levels of the donor and acceptor are almost isoenergetic for PC60BM (implying virtually zero driving force for charge separation) and this is reflected in inefficient charge photogeneration. A ketolactam fullerene with a deeper LUMO produces a higher level of charge photogeneration. Interestingly, it was discovered that the INDT polymers may possibly generate an intramolecular CT state-like singlet exciton, which is only able to be efficiently separated in the presence of a fullerene with a deep enough LUMO.

Electron-hole recombination determines photocurrent generation yields in polymer:fullerene blends, but the nature of this process and its timescales are not completely understood. In this study, we use a combination of spectroscopy techniques to probe how film structure and interface energetics control the charge generation and recombination dynamics in several polymer:fullerene blends. By varying film composition and comparing between different LUMO-LUMO offset systems, we identify geminate electron-hole recombination on the nanosecond timescale only in the blends consisting of finely-intermixed polymer:fullerene phases and low LUMO-LUMO offsets, whereas the formation of pure fullerene phases leads to the suppression of the geminate recombination process. Charge transfer state photoluminescence and electroluminescence data show similar dependencies confirming that geminate electron-hole recombination is controlled by interfacial enthalpic energy offsets and the density ratio between pure-fullerene and intermixed polymer-fullerene phases. Our results also indicate that electron-hole association probed with electroluminescence and photoluminescence derives from different interfaces playing distinct roles in the photocurrent generation process.

One of the key areas of study in organic photovoltaics is the development of so-called 'non-fullerene acceptors' (NFAs), which enjoy several benefits over older, fullerene-based acceptors, such as low cost, high absorptivity, and tuneability. A recent report Fei et. al. demonstrated conversion efficiencies of 13% in donor-acceptor blends comprising a fluorinated derivative of the common donor PBDB-T and an alkylated derivative of the ITIC (C8-ITIC) acceptor species. Understanding the underlying dynamics of this material is therefore important for the rational design of new NFAs. In order to understand the photophysical processes in C8-ITIC, we performed ultrafast transient absorption studies of four donor-acceptor blends, containing various combinations of C8-ITIC, PFBDB-T, and their unmodified predecessors. Long-lived excitons form at the acceptor regardless of the excitation frequency, suggestive of rapid energy transfer from the donor to the acceptor. Exciton decay at early times was more rapid in C8-ITIC compared to non-alkylated ITIC. A distinct change in exciton decay characteristics was observed at longer timescales in tandem with spectral drift in the acceptor’s excitonic peak. We use global analysis and a broader array of ultrafast spectroscopic techniques to elucidate the identity and mechanism behind this feature. Our results will help to shed light on the efficiency of this material and aid the development of more efficient and effective non-fullerene acceptors.

The interaction of organic semiconductors with confined light fields offers one of the easiest means to tune their material properties. In the regime of strong light-matter coupling, the semiconductor exciton and cavity photon mode hybridize to form new 'polariton' states. In organic systems these light-matter hybrids are tuneably separated by as much as 100’s of meV from the parent exciton, enabling radical alteration of the energetic landscape. The effects of strong coupling can be profound, including reports of long-range energy transfer, enhanced carrier mobility and altered chemical reactivity. Theoretical work is now increasingly focused on the potential of polariton to manipulate electronic dynamics in the excited state, but experimental realisation has proved challenging. Here, we demonstrate the ability to manipulate triplet photophysics in singlet exciton fission materials in the strong coupling regime. Within microcavities, we dramatically enhance the emission lifetime and increase delayed fluorescence by >100%, which we explain through a shift in the thermodynamic equilibrium between dark states in the exciton reservoir and the bright polaritons. Indeed, with this approach we can create entirely new radiative pathways, turning completely dark states bright and opening new scope for microcavity-controlled materials.

The negligible spin orbit coupling in many organic molecules creates opportunities to alter the energy of excited electrons by manipulating their spin. In particular, molecules with a large exchange splitting have garnered interest due to their potential to undergo singlet fission (SF), a process where a molecule in a high-energy spin-singlet state shares its energy with a neighbor, placing both in a low-energy spin-triplet state. When incorporated into photovoltaic and photocatalytic systems, SF can offset losses from carrier thermalization, which account for ~50% of the energy dissipated by these technologies. Likewise, compounds that undergo SF’s inverse, triplet fusion (TF), can be paired with infrared absorbers to create hybrid structures that upconvert infrared into visible light. However, integrating materials that undergo SF or TF with existing electronics remains challenging as the efficacy of these processes depends strongly on how molecules order in the solid state. I will summarize work aimed at identifying critical structure-function relationships that guide SF within perylenediimide (PDI) films. By adding functional groups at key locations along the PDI backbone, we can force these molecules to adopt different structures in the solid state. Guided by electronic structure calculations, we have used this approach to optimize the electronic coupling between PDIs such that they undergo SF with near quantitative efficiency. In addition, I will describe recent work whose goal is to understanding energy transfer pathways that operate in hybrid organic:inorganic structures that use TF to upconvert infrared light into the visible range.

Fluorescent conjugated polymers are attractive materials to produce low-cost and lightweight displays, lighting, and organic electronics. However, when transitioning from solution to solid state, maintaining the desired emissive properties of these materials remains a challenge; the emission wavelength and quantum yield of fluorescent polymers are highly sensitive to their solid state packing arrangements which are difficult to control or predict. Additionally, their susceptibility to photo-degradation limits their widespread use. Aggregation of the polymer can protect the material from most oxidative damage by reducing the diffusivity of the oxygen through the aggregate structure. Here we employ various bulk and single molecule fluorescence-based methods to explore this aspect of a well-studied organic semi-conductor, poly(3-hexylthiophene) (P3HT). Pre-aggregating P3HT with highly-polar solvents prior to spin casting leads to aggregate structures and thin films with significantly enhanced emissive intensity and photo-stability relative to films cast without pre-aggregation. Additionally, enhanced photo-oxidative stability was seen in films formed from the pre-aggregated samples. A better understanding of aggregate properties should lead to better control and higher performance of organic semiconductors in device applications.

In semiconductors in the high excitation density limit, inter-particle correlations and exchange forces increase to a point where the thermal and Fermi pressure are overcome. In this limit, electrons and holes condense to form a two component liquid phase. This new phase is determined by strong-coupling and quantum correlations and best described as a degenerate Fermi-liquid. An adequate description of this exotic state of matter lies at the intersection of plasma, solid-state, and quantum condensed matter physics. Many of the properties of this condensed phase would find great use in the semiconductor applications, e.g. high-speed optoelectronic transistors, semimetal conductivity, broadband light emission and amplification, high droplet mobility. However, due to material parameters the observation of EHL state is mostly limited to cryogenic temperatures, and thus practical applications are hindered. The critical temperature "T" under which EHL exists (gas-liquid transition), is empirically found to be approximately one tenth of the exciton binding energy "E_b ". Since most semiconductors exhibit high dielectric screening (ϵ≳10), typical values for E_b range from 1-100 meV, therefore the EHL state is observed typically below 100K. Semiconductors with higher binding energies are being explored to reach ever-higher values of T. With Eb=80 meV, diamond represents the current state-of-the-art material to show EHL at TL=165 K. Reaching room temperature condensation will require a significant reduction in the dielectric screening. In that regard atomically thin 2D materials, provides a significant opportunity to push condensation to room temperature values, due to their reduced dimensionality and weakened material screening. These materials exhibit significantly increased excitonic binding energies as well as substantially high electron and hole effective masses, both of which favor condensation. Here we by using time resolved and steady state photoluminescence, differential absorption, and Raman spectroscopies, we investigated the formation and dissipation dynamics of EHL in 2D MoS2. We will discuss the roll of electronic and structural properties of the single layer materials in hosting such high excitation density states.

Layered transition metal dichalcogenides (TMDs) represent a diverse, emerging source of two-dimensional (2D) nanostructures with broad application in optoelectronics and energy. In particular, tungsten disulfide (WS₂) is an efficient visible light absorber with relatively high carrier mobilities and catalytic activity towards hydrogen evolution. While explanation of the quantum confinement and excitonic effects governing TMD optoelectronic properties has progressed in recent years, less is known about the ultra-fast photoresponse and carrier dynamics following light excitation. This work utilizes transient absorption spectroscopy, with pump tunability and broadband visible probing, to monitor the carrier dynamics of both CVD-grown monolayer and solution exfoliated WS₂. Picosecond-scale features include simultaneous bleaching of excitonic states and a red-shifted absorption spectrum attributed to bandgap renormalization, while free carriers, defect trapped carriers, and recombination signatures are apparent at increasing pico- to nanosecond lifetimes. Features associated with excitons, trions, and photo-excited carriers exhibit strong dependence on local environmental factors. Moisture, oxygen, chemical dopants, and dielectric environment strongly affect the strength and decay lifetimes of these photo-excited species. These results highlight the importance of understanding and controlling these local environmental factors to the rational design and implementation of 2D TMDs optoelectronic device platforms.

Surface plasmon resonant nanoantennas can confine incident energy onto two-dimensional (2D) transition metal dichalcogenides (TMD) to enhance efficiency of harmonic conversion to higher energies, which is otherwise limited by the intrinsic Å-scale interaction length. Second harmonic generation (SHG) from nanoantenna-decorated 2D TMD was heuristically examined with hyper Rayleigh scattering (HRS), multi-photon microscopy, electron energy loss spectroscopy (EELS), and discrete dipole computation. HRS experimentally quantified the frequency dependence of the second-order nonlinear susceptibility, χ ⁽²⁾ , for liquid-exfoliated WS₂. Measured χ⁽²⁾ fell within 21% of independent density functional theory (DFT) calculations, overcoming the known 100-1000x overestimation of microscopy approaches. EELS supported design of nanoantennas for integration with TMD. Overall SHG conversion efficiencies from chemical vapor-deposited (CVD) 4×10⁵ nm² MoS₂ crystals on silicon dioxide were enhanced up to 0.025 % W^-1 in the presence of by single 150 nm Au nanoshell monomers and dimers, ostensibly due to augmented local plasmonic fields.

The electronic structure of interfaces comprising electronic materials governs fundamental charge and energy transfer processes, and thus the functionality and efficiency of electronic and optoelectronic devices featuring these interfaces. Mixed cation and halide perovskites are considered prime materials for photovoltaics. Yet, the fundamental understanding of their electronic structure and the energy level alignment with charge transport layers is limited. As is discussed in this contribution, perovskite surface states and concomitant surface photovoltage effects can mask the ground state electronic properties in photoemission experiments, and only low photon dose procedures allow unraveling reliable interface energetics of relevance for devices. Two-dimensional (2D) transition metal dichalcogenides (TMD) semiconductors also emerge as highly interesting electronic materials. They feature direct energy gaps in monolayer form, and their pronounced excitonic nature offers the possibility of fine-tuning electronic and optical properties by engineering the dielectric environment. As exemplified here, the exciton binding energy of MoS2 and WSe2 can vary by a factor of two, depending on the substrate’s dielectric constant. Furthermore, charge transfer interactions with molecular electron donors and acceptors facilitate doping of TMDs. The mechanism of this type of interface doping is contrasted with that of conventional semiconductor (GaN, Si) surfaces. For the latter case, it is demonstrated how molecular donors and acceptors can be employed to tune the level alignment at inorganic/organic semiconductor heterojunctions over extreme intervals.

Though bulk silicon (Si) is an indirect bandgap material and therefore non-emissive, nm-sized Si quantum dots (Si QDs) exhibit direct band-gap characteristics due to quantum confinement. As a result, Si QDs are emissive though their fluorescence is relatively weak and limited to red or near-infrared. Recently, visible and color-tunable emission with up to 90% quantum yield has been achieved through surface-modification of Si QDs with nitrogen-capped ligands. However, the emission mechanism operating in these surface-modified Si QDs is unclear and the factors that determine their emission maxima are still unknown. Here we report that the emission maximum wavelength of these species can be predicted quantitatively from the calculated ground-state dipole moment of the ligand. This is consistent with the origin of the emission being a charge-transfer (CT) transition between the Si surface and the ligand. A detailed study of the photon statistics behavior of isolated Si QDs reveals two types of emission, the dominant one being characteristic of single quantum states and the weaker one being characteristics of a bulk material. Understanding the emission mechanism of these unique systems and how their properties can be tuned synthetically will enable the design of Si QDs with a broader wavelength range and higher quantum yields for applications in light-emitting diodes, bio-imaging and sensing.

Quantum dots (QDs)-based fluoresecnce resonance energy transfer (FRET) processes are implemented to develop a variety of optical sensors. Herein we report the construction of dual-colored QDs-based binary FRET (BiFRET) systems and demonstrate their feasibility for ratiometric detection of Zn2+. The QD-FRET system was constructed by first coating of spacer on the QD donor, followed by loading of multiple acceptors, meso-tetra(4-sulfonatophenyl)porphine dihydrochloride (TSPP), on the spacer. The spacer thickness controlled the QD-TSPP FRET distance, determining the FRET efficiency. Silica, DNA and poly(dA) were used, respectively, to control the QD-TSPP FRET distance and demonstrated different FRET efficiencies. The biocopolymer poly(dA) had the minimal space thickness of 1.7 nm, achieving the highest QD-TSPP FRET efficency of 70%. By mixing two QD-FRET systems, a QD-BiFRET system (FRET-1 for 517QD-TSPP, FRET-2 for 560QD-TSPP) was facilely prepared, which could be used for Zn2+ detection. In the absence of Zn2+, FRET-1 is efficient while FRET-2 is inefficient. In the presence of Zn2+, the chelation of Zn2+ changed the TSPP absorption at 515 nm and 555 nm in an oposite manner. Accordingly, FRET-1 became inefficient and FRET-2 became efficient. As a result, the QD-BiFRET system realized ratiometric detection of Zn2+. The system with the poly(dA) spacer had obtained the detection limit of 1 nM, which is sixtieth part of that of the system with the silica spacer. This work is financially supported by NSFC (Grant: 21775021).

Semiconductor quantum dots (QDs) are emerging luminescent nanomaterials. It can be used in solid-state lighting (SSL), display, solar cells and biomedical imaging due to the exhibit excellent wavelength tunability, large excitation range, consistent particle size and high quantum efficiency. In solid state lighting, white light-emitting diodes (LEDs) are often made by using QDs such as CdSe or ZnCdSe. However, cadmium-based QDs have limited future applications owing to the well-known toxicity. Recently, cadmium-free luminescent materials-CuInS₂/ZnS (CIS/ZnS) core/shell QDs are investigated. The CIS/ZnS QDs exhibit very broad emission spectrum, large Stoke’s shift, and tunable emission wavelengths. Those properties make the CIS/ZnS QDs suitable for solid-state lighting application. In this study, CIS QDs with molar ratio of Cu:In is equal to 1:4, and the ZnS shell was produced by different shell sulfide precursors, such as dodecanethiol (DDT), octadecanethiol (ODT) and sulfur (S) powders. The optical properties, morphologies, and crystal structure are analysis by fluorescence spectrometer, UV-Vis spectrometer, transmission electron microscopy, and X-ray diffractometer, respectively. The results show that the emission wavelength and quantum yield (QY) of CIS/ZnSDDT, CIS/ZnSODT, and CIS/ZnSS are 549 nm, 76 %, 548 nm, 82 %, and 538 nm, 83 %, respectively. The structure of CIS/ZnS QD belongs to chalcopyrite phase and the average particle size is 3.2 nm. Moreover, the stability of CIS/ZnS QDs is excellent.

We describe the necessary steps towards the realization of ultrafast revealing of invisible patterns encrypted in colloidal photonic crystals. These include the development of hollow air-core – dense-silica-shell core-shell monodisperse and spherical nanoparticles; introducing of a pattern of hydrophilic regions in a hydrophobic surrounding; and the combination of these two approaches by selective oxygen plasma etching of hollow core-shell nanospheres. The pattern imprinted by the difference in only surface property remains invisible in normal conditions of static environmental humidity. The hydrophilic regions in the patterns are reversible and immediately unveiled by dynamic humid flow. The specific properties of a human breath in terms of relative humidity and vapor flow are ideal for optimal revealing in terms of the spectral shift of the photonic bandgap of the colloidal crystal. The revealing of the pattern is determined by the surface tension of the vapor, while the color of the imprinted pattern is independently determined by its refractive index.