Probing Charge Transfer Research Articles

Probing Charge Transfer across Various Metal Support Interactions in Heterogeneous Catalysts via 4D-STEM Techniques

Probing Charge Transfer across Various Metal Support Interactions in Heterogeneous Catalysts via 4D-STEM Techniques

Probing charge transfer of NiCo2O4/g-C3N4 photocatalyst for hydrogen production

Quantum dots/two-dimensional (0D/2D) semiconductor photocatalysts demonstrate wide solar light absorption region and high charge transfer efficiency. However, the relation between the interfacial electric field and the charge transfer during the photocatalytic hydrogen production process is still unclear. Here, we construct NiCo2O4 quantum dots (QDs) and NiCo2O4 nanoparticles (NPs) anchored with 2D g-C3N4 (CN) to form NiCo2O4-QDs/CN and NiCo2O4-NPs/CN heterojunctions. The hydrogen production rate of CN loaded with NiCo2O4 QDs is about 3 times higher than that of CN loaded with NiCo2O4 NPs. The electric field intensity at the NiCo2O4-QDs/CN interface is calculated to be about 15,600 V cm−2, about 9 times higher than that of NiCo2O4-NPs/CN, which could effectively drive the electrons of CN to flow toward NiCo2O4 QDs, promoting photocarriers separation and hence greatly improving the photocatalytic performance. This work provides a method to understand the relationship between interfacial electric field strength and photogenerated charges of heterostructure photocatalysts.

Probing Charge Transfer Character in Modern Donor/Acceptor Materials via Electroabsorption Spectroscopy.

We use electroabsorption (EA) spectroscopy to probe the charge transfer (CT) character in neat films and blends of donors and acceptors of interest for organic electronic applications. In particular, we compare the CT character in two polymer donor and non-fullerene acceptor blends, including 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (IT-4F) and 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6). Like classic polymer/fullerene blends, the blend based on IT-4F exhibits primarily first derivative-like EA features, suggesting localized exciton formation upon photoexcitation. However, the Y6-based blend has an EA spectrum that is dominated by second derivative-like features, consistent with CT character. We show that this signal originates primarily from Y6. We find that Y6 exhibits the highest dipole moment change (7.5 ± 2.5 D) of the molecules that comprise this study, consistent with a high degree of the CT character, and a relatively large polarization volume of 361 ± 70 Å3, consistent with strong electron delocalization. These results point to the origins of exceptional performance of organic photovoltaics (OPVs) based on Y6.

Probing charge transfer through antifouling polymer brushes by electrochemical methods: The impact of supporting self-assembled monolayer chain length

Ultrathin surface-tethered polymer brushes represent attractive platforms for a wide range of sensing applications in strategically vital areas such as medicine, forensics, or security. The recent trends in such developments towards “real world conditions” highlighted the role of zwitterionic poly(carboxybetaine) (pCB) brushes which provide excellent antifouling properties combined with bio-functionalization capacity. Highly dense pCB brushes are usually prepared by the “grafting from” polymerization triggered by initiators on self-assembled monolayers (SAMs). Here, multi-methodological experimental studies are pursued to elucidate the impact of the alkanethiolate SAM chain length (C6, C8 and C11) on structural and functional properties of antifouling poly(carboxybetaine methacrylamide) (pCBMAA) brush. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a custom-made 3D printed cell employing [Ru(NH3)6]3+/2+ redox probe were used to investigate penetrability of SAM/pCBMAA bilayers for small molecules and interfacial charge transfer characteristics. The biofouling resistance of pCBMAA brushes was characterized by surface plasmon resonance; ellipsometry and FT-IRRAS spectroscopy were used to determine swelling and relative density of the brushes synthesized from initiator-bearing SAMs with varied carbon chain length. The SAM length was found to have a substantial impact on all studied characteristics; the highest value of charge transfer resistance (Rct) was observed for denser pCBMAA on longer-chain (C11) SAM when compared to shorter (C8/C6) SAMs. The observed high value of Rct for C11 implies a limitation for the analytical performance of electrochemical sensing methods. At the same time, the pCBMAA brushes on C11 SAM exhibited the best bio-fouling resistance among inspected systems. This demonstrates that proper selection of supporting structures for brushes is critical in the design of these assemblies for biosensing applications.

Second Harmonic Generation in van der Waals Heterostructure of Centrosymmetric ReS2 and Graphene

AbstractVan der Waals (vdW) heterostructures of two‐dimensional (2D) materials promise exotic properties beyond the reach of existing material systems and open unprecedented opportunities for device applications. Here, it is demonstrated that vdW stacking of centrosymmetric ReS2 and graphene into heterostructure could induce strong second harmonic generation (SHG), though there is not any SHG from the individual ReS2 or graphene. It could be attributed to the interfacial charge transfer in the heterostructure. With the distorted 1T crystal structure of ReS2 and electrostatic screening effect, the charge transfer gives rise to non‐uniform charge distribution across the ReS2's atomic layers, resulting in the broken centrosymmetry for the second‐order hyperpolarizability. At the specific pump wavelength of 1550 nm, the strength of the induced SHG in a trilayer ReS2/graphene vdW heterostructure is approximately twice of that from a monolayer MoS2. This work reveals vdW stacking is a simple and efficient method for inducing SHG in 2D materials with centrosymmetry, which could be considered as another unique attribute of 2D materials. It also indicates that SHG spectroscopy is a valid technique for probing charge transfer and distribution in heterostructures of 2D materials.

Probing charge transfer under external bias at Cu/SrTiO3 heterojunction

Supported metal–metal oxide heterojunctions are ubiquitous in designing highly efficient catalysts for energy conversion and storage devices. The interaction between metal and supports has attracted significant interest in developing novel catalysts. Especially, surface charge distribution and charge transfer between metallic particles and oxide supports are vital for understanding the catalytic reaction. Here, we investigate the effect of the external electric field and circumstances on the charge transfer within the Cu/SrTiO3 heterojunction. Based on the in-situ Kelvin probe force microscopy (in-situ KPFM) and density functional theory (DFT) calculations, we observe and analyze that the specific behavior of charge transfer could be switched by reversing the external electric field, indicating the role of Cu microparticles could be altered electrically between donor and acceptor. Moreover, the circumstances surrounding the Cu/SrTiO3 heterojunction influenced the transfer of charge as well. We also present a comprehensive model of the observed behavior based on work functions and electronic structure of Cu/SrTiO3 heterojunction. Our work provides insight into the charge transfer in metal–semiconductor heterojunction and is helpful to design highly active heterostructured catalysts.

Manipulating and Probing the Distribution of Excess Electrons in an Electrically Isolated Self-Assembled Molecular Structure.

Exploiting single electrical charges and their mutual interactions for computation has been proposed as a concept for future nanoelectronics. Controlling and probing charge transfer in electrically isolated atomic-scale structures are fundamental to push its experimental realization. Here, we controllably inject individual excess electrons and study their distribution in a self-assembled structure supported on a nonconductive substrate. The self-assembly ensures structural order down to the atomic scale. Depending on the charge state of the molecular assembly, intermolecular electron hopping and specific electron distributions have been resolved by atomic force microscopy, clarifying the charge-transfer pathways in the tunnel-coupled structure. When mutual charge interactions were exploited, control over specific charge distributions in the self-assembled structure has been achieved with single-molecule precision, paving the way toward the design of data processing platforms based on molecular nanostructures.

Probing charge transfer processes at p-GaAs electrodes under weak optical excitation

The p-GaAs photocathode/acetonitrile interface was characterised in the dark and with tens of μW/cm2 laser powers at 830 nm wavelength, using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Two redox mediators were used that present completely different behaviours under low optical excitation: one (Tris(2,2’-bipyridine)ruthenium(II) hexafluorophosphate) is sensitive to the surface states of the photo-cathode and the other (benzoquinone) is not. The combined electrochemical methods allow us to characterize the energetics of the electrode-electrolyte interface as well as the dominant electron transfer process.

Electroabsorption Spectroscopy as a Tool for Probing Charge Transfer and State Mixing in Thermally Activated Delayed Fluorescence Emitters.

Solid-state electroabsorption is demonstrated as a powerful tool for probing the charge transfer (CT) character and state mixing in the low-energy optical transitions of two structurally similar thermally activated delayed fluorescent (TADF) materials with divergent photophysical and device performances. The Liptay model is used to fit differentials of the low-energy absorption bands to the measured electroabsorption spectra, with both emitters showing CT characteristics and large changes in dipole moments upon excitation despite the associated absorption bands appearing to be structured. High electric fields then reveal transfer of oscillator strength to a state close to the CT in the better performing molecule. With supporting TDDFT-TDA and DFT/MRCI calculations, this state showed ππ* characteristics of a local acceptor triplet that strongly mixes with the σπ* of the CT. The emitter with poor TADF performance showed no evidence of such mixing.

Probing charge transfer states at organic and hybrid internal interfaces by photothermal deflection spectroscopy

In organic and hybrid photovoltaic devices, the asymmetry required for charge separation necessitates the use of a donor and an acceptor material, resulting in the formation of internal interfaces in the device active layer. While the core objective of these interfaces is to facilitate charge separation, bound states between electrons and holes may form across them, resulting in a loss mechanism that diminishes the performance of the solar cells. These interfacial transitions appear in organic systems as charge transfer (CT) states and as bound charge pairs (BCP) in hybrid systems. Despite being similar, the latter are far less investigated. Herein, we employ photothermal deflection spectroscopy and pump-push-probe experiments in order to determine the characteristics and dynamics of interfacial states in two model systems: an organic P3HT:PCBM and hybrid P3HT:ZnO photovoltaic layer. By controlling the area of the internal interface, we identify CT states between 1.4 eV and 1.8 eV in the organic bulk-heterojunction (BHJ) and BCP between 1.1 eV and 1.4 eV in the hybrid BHJ. The energetic distribution of these states suggests that they not only contribute to losses in photocurrent, but also significantly limit the possible maximum open circuit voltage obtainable from these devices.

Probing charge transfer during metal-insulator transitions in graphene-LaAlO3/SrTiO3 systems

Two-dimensional electron systems (2DESs) at the interface between LaAlO3 (LAO) and SrTiO3 (STO) perovskite oxides display a wide class of tunable phenomena ranging from superconductivity to metal-insulator transitions. Most of these effects are strongly sensitive to surface physics and often involve charge transfer mechanisms, which are, however, hard to detect. In this work, we realize hybrid field-effect devices where graphene is used to modulate the transport properties of the LAO/STO 2DES. Different from a conventional gate, graphene is semimetallic and allows us to probe charge transfer with the oxide structure underneath the field-effect electrode. In LAO/STO samples with a low initial carrier density, graphene-covered regions turn insulating when the temperature is lowered to 3 K, but conduction can be restored in the oxide structure by increasing the temperature or by field effect. The evolution of graphene’s electron density is found to be inconsistent with a depletion of LAO/STO, but it rather points to a localization of interfacial carriers in the oxide structure.

Probing charge transfer dynamics in a single iron tetraphenylporphyrin dyad adsorbed on an insulating surface.

Although the dynamics of charge transfer (CT) processes can be probed with ultimate lifetime resolution, the inability to control CT at the nanoscale is one of the most important roadblocks to revealing some of its deep fundamental aspects. In this work, we present an investigation of CT dynamics in a single iron tetraphenylporphyrin (Fe-TPP) donor/acceptor dyad adsorbed on a CaF2/Si(100) insulating surface. The tip of a scanning tunneling microscope (STM) is used to create local ionic states in one fragment of the dyad. The CT process is monitored by imaging subsequent changes in the neighbor acceptor molecule and its efficiency is mapped, revealing the influence of the initial excited state in the donor molecule. In the validation of the experiments, simulations based on density functional theory show that holes have a higher donor-acceptor CT rate compared to electrons and highlight a noticeable initial state dependence on the CT process. We leverage the unprecedented spatial resolution achieved in our experiments to show that the CT process in the dyad is governed via molecule-molecule coherent tunneling with negligible surface-mediated character.

Probing charge transfer characteristics in a donor-acceptor metal-organic framework by Raman spectroelectrochemistry and pressure-dependence studies.

The stimuli responsive behaviour of charge transfer donor-acceptor metal-organic frameworks (MOFs) remains an understudied phenomenon which may have applications in tuneable electronic materials. We now report the modification of donor-acceptor charge transfer characteristics in a semiconducting tetrathiafulvalene-naphthalene diimide-based MOF under applied electrochemical bias and pressure. We employ a facile solid state in situ Raman spectroelectrochemical technique, applied for the first time in the characterisation of electroactive MOFs, to monitor the formation of a new complex TTFTC˙+-DPNI from a largely neutral system, upon electrochemical oxidation of the framework. In situ pressure-dependent Raman spectroscopy and powder X-ray diffraction experiments performed in a diamond anvil cell revealed blue shifts in the donor and acceptor vibrational modes in addition to contractions in the unit cell which are indicative of bond shortening. This study demonstrates the utility of in situ Raman spectroscopic techniques in the characterisation of redox-active MOFs and the elucidation of their electronic behaviours.

Probing charge transfer between molecular semiconductors and graphene

The unique density of states and exceptionally low electrical noise allow graphene-based field effect devices to be utilized as extremely sensitive potentiometers for probing charge transfer with adsorbed species. On the other hand, molecular level alignment at the interface with electrodes can strongly influence the performance of organic-based devices. For this reason, interfacial band engineering is crucial for potential applications of graphene/organic semiconductor heterostructures. Here, we demonstrate charge transfer between graphene and two molecular semiconductors, parahexaphenyl and buckminsterfullerene C60. Through in-situ measurements, we directly probe the charge transfer as the interfacial dipoles are formed. It is found that the adsorbed molecules do not affect electron scattering rates in graphene, indicating that charge transfer is the main mechanism governing the level alignment. From the amount of transferred charge and the molecular coverage of the grown films, the amount of charge transferred per adsorbed molecule is estimated, indicating very weak interaction.

Probing Charge Transfer and Hot Carrier Dynamics in Organic Solar Cells with Terahertz Spectroscopy.

Time-resolved terahertz spectroscopy (TRTS) was used to explore charge generation, transfer, and the role of hot carriers in organic solar cell materials. Two model molecular photovoltaic systems were investigated: with zinc phthalocyanine (ZnPc) or alpha-sexathiophene (α-6T) as the electron donors and buckminsterfullerene (C60) as the electron acceptor. TRTS provides charge carrier conductivity dynamics comprised of changes in both population and mobility. By using time-resolved optical spectroscopy in conjunction with TRTS, these two contributions can be disentangled. The sub-picosecond photo-induced conductivity decay dynamics of C60 were revealed to be caused by auto-ionization: the intrinsic process by which charge is generated in molecular solids. In donor-acceptor blends, the long-lived photo-induced conductivity is used for weight fraction optimization of the constituents. In nanoscale multilayer films, the photo-induced conductivity identifies optimal layer thicknesses. In films of ZnPc/C60, electron transfer from ZnPc yields hot charges that localize and become less mobile as they thermalize. Excitation of high-lying Franck Condon states in C60 followed by hole-transfer to ZnPc similarly produces hot charge carriers that self-localize; charge transfer clearly precedes carrier cooling. This picture is contrasted to charge transfer in α-6T/C60, where hole transfer takes place from a thermalized state and produces equilibrium carriers that do not show characteristic signs of cooling and self-localization. These results illustrate the value of terahertz spectroscopic methods for probing charge transfer reactions.

Probing charge transfer in a novel class of luminescent perovskite-based heterostructures composed of quantum dots bound to RE-activated CaTiO3 phosphors.

We report on the synthesis and structural characterization of novel semiconducting heterostructures composed of cadmium selenide (CdSe) quantum dots (QDs) attached onto the surfaces of novel high-surface area, porous rare-earth-ion doped alkaline earth titanate micron-scale spherical motifs, i.e. both Eu-doped and Pr-doped CaTiO3, composed of constituent, component nanoparticles. These unique metal oxide perovskite building blocks were created by a multi-pronged synthetic strategy involving molten salt and hydrothermal protocols. Subsequently, optical characterization of these heterostructures indicated a clear behavioral dependence of charge transfer in these systems upon a number of parameters such as the nature of the dopant, the reaction temperature, and particle size. Specifically, 2.7 nm diameter ligand-functionalized CdSe QDs were anchored onto sub-micron sized CaTiO3-based spherical assemblies, prepared by molten salt protocols. We found that both the Pr- and Eu-doped CaTiO3 displayed pronounced PL emissions, with maximum intensities observed using optimized lanthanide concentrations of 0.2 mol% and 6 mol%, respectively. Analogous experiments were performed on Eu-doped BaTiO3 and SrTiO3 motifs, but CaTiO3 still performed as the most effective host material amongst the three perovskite systems tested. Moreover, the ligand-capped CdSe QD-doped CaTiO3 heterostructures exhibited effective charge transfer between the two individual constituent nanoscale components, an assertion corroborated by the corresponding quenching of their measured PL signals.

Probing charge transfer dynamics in self-assembled monolayers by core hole clock approach

This article reviews recent progress in the application of core hole clock approach in the framework of resonant Auger electron spectroscopy to the monomolecular assembles of alkyl, oligophenyl, and oligo(phenylene–ethynylene) based molecules on Au(111) substrates, referring mostly to the work by the author et al. The major goal was to study electron transfer (ET) dynamics in these systems serving as prototypes of molecular electronics (ME) devices. The ET pathway to the conductive substrate was unambiguously defined by resonant excitation of the nitrile tailgroup attached to the molecular backbone. Characteristic ET times within the femtosecond domain were determined, along with the attenuation factors for the ET dynamics, analogous to the case of the static transport. The above parameters were found to exhibit strong dependence on the character of the molecular orbital which mediates the ET process. In addition, certain spectral features, which can be associated with an inverse ET from the molecular backbone to the excitation site, were observed upon exchange of the nitrile group by strongly electronegative nitro moiety. The reported results represent a valuable input for theory and a certain potential for applications such as ME devices where optimization of ET can have significant technological impact.

Probing Charge Transfer in Organic Photovoltaic Cells by Directly Exciting Single Electron

Probing Charge Transfer in Organic Photovoltaic Cells by Directly Exciting Single Electron

Probing Charge Transfer between Shells of Double‐Walled Carbon Nanotubes Sorted by Outer‐Wall Electronic Type

Double-walled carbon nanotubes (DWCNTs) with outer metallic (M) or semiconducting (S) shells were sorted by density-gradient ultracentrifugation and examined by Raman spectroscopy and in situ Raman spectroelectrochemistry. The combination of sorting and the selection of appropriate laser excitation energies allowed the disentanglement of the effects of different variations of the electronic type (M or S) of the inner and outer tubes in DWCNTs on the doping behavior and charge transfer between the inner and outer walls. Charge transfer from the outer tube to the inner tube occurs only if the electronic states of the outer tube are filled with electrons or holes, and if these filled states are higher in energy than those of the inner tube. Therefore, each combination of inner and outer tube (i.e., inner@outer: M@M, M@S, S@M, and S@S) exhibits a distinct behavior. The potential needed to observe the effects of charge transfer between the inner and outer tubes is found to increase in the following order: M@M < S@M < M@S < S@S.

Probing Charge Transfer at Surfaces Using Graphene Transistors

Graphene field effect transistors (FETs) are extremely sensitive to gas exposure. Charge transfer doping of graphene FETs by atmospheric gas is ubiquitous but not yet understood. We have used graphene FETs to probe minute changes in electrochemical potential during high-purity gas exposure experiments. Our study shows quantitatively that electrochemistry involving adsorbed water, graphene, and the substrate is responsible for doping. We not only identify the water/oxygen redox couple as the underlying mechanism but also capture the kinetics of this reaction. The graphene FET is highlighted here as an extremely sensitive potentiometer for probing electrochemical reactions at interfaces, arising from the unique density of states of graphene. This work establishes a fundamental basis on which new electrochemical nanoprobes and gas sensors can be developed with graphene.