electrode interfaces studied by core-hole clock spectroscopy.

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Quantitative Femtosecond Charge Transfer Dynamics at Organic/Electrode Interfaces Studied by Core-Hole Clock Spectroscopy Liang Cao, Xing-Yu Gao, Andrew T. S. Wee,* and Dong-Chen Qi* progress in the performance of both materials and devices. For example, even polymer semiconductor thin films, processed from solution, have demonstrated charge carrier mobility in excess of 10 cm2V−1s−1 in the last year or so,[1] making it highly conceivable to use transistors printed with organic semiconductors as backplane driving circuits for flexible electronic devices. Organic semiconductor materials exhibit many advantages including low cost, low temperature and solution processability, tremendous structural flexibility, and synthetic tailorability. The electronic and optical properties of organic semiconductors can be dramatically modified by proper functionality.[2] Such functional organic molecules are widely applied in devices such as organic photovoltaic cells (OPVCs),[3] organic lightemitting diodes (OLEDs),[4] organic fieldeffect transistors (OFETs),[5] dye sensitized solar cells (DSSCs)[6] and organic memory (OMEM).[7] Many of these technologies have already reached or are close to market applications in areas like displays, flexible solar panels or integrated circuits. In addition, extensive research has been devoted to developing novel applications such as bio sensors,[8] and spintronics,[9] aiming to realize the full potential of organic semiconductor materials.[10] Almost all organic electronic device architectures involve interfaces formed between organic materials and electrodes, and the interfacial charge transfer process, such as charge injection/extraction at the molecule-metal interface in OLEDs/ OPVCs, electron extraction in DSSCs, electron transport across metal-molecule junction, plays an important role in determining the device performance.[11] Understanding the charge transfer dynamics and quantifying the relevant timescale not only provide fundamental insights into the electronic processes in organic molecules and their interfaces, but also serve as an important gauge for the rationale design and engineering of organic interfaces at the molecular scale. Most time-resolved experimental techniques to probe ultrafast dynamic processes rely on pump-probe methods, in which dynamics is firstly initiated by a short laser “pump” pulse and the evolution of the dynamics is then probed by a second “probe” pulse after a variable time delay. The time resolution of pump-probe measurements is therefore limited by both the pulse widths and the time delay between the two pulses.[12] Over the past decade, tremendous progress has been achieved

Organic semiconductor materials have important applications in organic electronics and other novel hybrid devices. In these devices, the transport of charge carriers across the interfaces between organic molecules and electrodes plays an important role in determining the device performance. Charge transfer dynamics at the organic/electrode interface usually occurs at the several femtoseconds timescale, and quantitative charge transfer dynamics data can been inferred using synchrotron-based core-hole clock (CHC) spectroscopy. In this research news, we have reviewed recent progress in the applications of CHC spectroscopy on the quantitative characterization of charge transfer dynamics at organic/electrode interfaces. By examining charge transfer dynamics at different types of interface, from weakly interacting van der Waals-type interfaces to interfaces with strong covalent bonds, we discuss a few factors that have been found to affect the charge transfer dynamics. We also review the application of CHC spectroscopy to quantify through-bonds and through-space charge transport in organic molecules.

1. Introduction Over the past few decades, organic electronics incorporating π-conjugated organic semiconducting molecules or polymers as active components have experienced hugely impressive Dr. L. Cao Department of Physics National University of Singapore 2 Science Drive 3, Singapore, 117542, Singapore Department of Chemistry National University of Singapore 3 Science Drive 3, Singapore 117543, Singapore Prof. X.-Y. Gao Department of Physics National University of Singapore 2 Science Drive 3, Singapore, 117542, Singapore; Shanghai Institute of Applied Physics Chinese Academy of Sciences, P. O. Box 800–204, Shanghai 201800, P. R. China Prof. A. T. S. Wee Department of Physics National University of Singapore 2 Science Drive 3, Singapore 117542, Singapore E-mail: [email protected] Dr. D.-C. Qi Department of Physics La Trobe University Victoria, Australia 3086 E-mail: [email protected]

DOI: 10.1002/adma.201305414

Adv. Mater. 2014, DOI: 10.1002/adma.201305414

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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in employing synchrotron-based core-hole clock (CHC) spectroscopy to probe the ultrafast charge transfer dynamics at organic/electrode interfaces. In contrast to the “direct” pump-probe measurements, CHC technique uses the intrinsic core-hole lifetime of a few femtoseconds (fs) as an internal reference clock to “indirectly” probe dynamic processes. Consequently, this technique can provide temporal information on the sub3-fs[13] or even attosecond[14] charge transfer dynamics, which is, to the best of our knowledge, superior to many pump-probe based methods in terms of temporal resolution.[12] For more detailed comparisons between the CHC technique and the laser-based pumpprobe technique, the reader is referred to the reviews by Menzel[15] and Zhu.[12a] It is worth mentioning that with the availability of Figure 1. Schematic overview of different photo-excitation and decay processes. Direct photosub-picosecond or even fs x-ray free electron emission from valence state (a) and core level (b). Resonant excitation of a core level electron laser (xFEL),[16] it is now possible to combine to the LUMOs (c) and the subsequent competitive core-hole decay processes of spectator xFEL with optical lasers to perform pump- decay (resonant Auger) (d), participator decay (resonant photoemission) (e) and charge probe measurements.[17] However, xFEL- transfer of excited electron to substrate conduction band (f). Normal Auger decay process based pump-probe measurements remain (g) following either (f) or (b) process. challenging, and the application of such involved in CHC technique. In the direct photoemission protechnique in the investigation of charge transfer dynamics at cess, the photoelectrons from valence state (Figure 1a) or core organic/electrode interfaces is scarce because (i) xFEL facilities level (Figure 1b) are excited to the vacuum and detected by the are far less accessbile than regular synchrotron radiation facilianalyzer. Figure 1c represents a typical x-ray absorption process ties; (ii) it is rather difficult to synchronize xFEL and regular in molecules, in which a core level electron (e.g., C 1s, N 1s) optical lasers; (iii) spectral features including shapes and peak is photo-excited to the lowest unoccupied molecular orbitals positions may be distorted by the space charge effect due to (LUMOs), creating a core-hole. Within the specific core-hole Coulomb repulsion between photoelectrons emitted in a ultralifetime of several fs for low Z elements, the autoionization short pulse.[18] Besides high temporal resolution, CHC specdecay process (Figure 1d and 1e) and charge transfer process troscopy is both elemental and orbital specific,[19] which allows (Figure 1f) compete with each other, and the faster process will studying charge transfer dynamics at specific atomic sites (or dominate. For system whereby the molecules are usually isofunctional groups) and unoccupied orbitals of organic molelated from the substrate such as in a multilayer, photo-excited cules. This is particular important because the electronic states electrons are localized in the LUMOs and not transferred into of organic semiconducting molecules can be rationally tuned by the substrate conduction band within the core-hole lifetime. either element substitution or insertion of particular functional The core-hole will predominantly decay through the autoionigroups, and selective studies of charge transfer dynamics for zation process, namely spectator decay (resonant Auger) and these substitutional elements and/or functional groups allow participator decay (resonant photoemission). In the spectator their influence on the performance of devices to be examined decay channel as shown in Figure 1d, the photo-excited elecexplicitly. Excellent and comprehensive reviews of the basic tron remains in the LUMOs in the final state and does not concepts of CHC spectroscopy and/or its qualitative applicadirectly participate in the de-excitation process, but indirectly tions at organic/inorganic interfaces can be found in refs.[19,20] influences the Auger–like decay process by increasing the The purpose of this paper is to review recent progress in the kinetic energy (KE) of Auger electrons through screening. application of CHC spectroscopy on the quantitative characteriConsequently, new resonant Auger features with constant in zation of charge transfer dynamics between organic molecules KE will emerge at the lower binding energy (BE) side of the and electrodes. normal Auger (non-resonant). Figure 1e shows the participator decay in which the resonantly excited electron fills the core2. Basic Principle of Core-Hole Clock Spectroscopy hole and a highest occupied molecular orbital (HOMO) electron is ejected by taking up the excess energy released by the decay process, leaving the system with a single HOMO hole. CHC spectroscopy allows the timescale measurement of the This final state is energetically equivalent to direct valence state delocalization of photo-excited electrons from unoccupied photoemission (c.f. Figure 1a) by the incident photon, but a molecular orbitals to substrate conduction bands. The principle larger cross section for the valence electron is achieved due to is briefly described below, and readers are referred to ref. [19] core-hole assisted resonant photoemission, resulting in a resfor a more comprehensive account. Figure 1 shows a schematic onant enhancement of the associated HOMO features in the overview of different photo-excitation and decay processes

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∞

t ⎛ t ⎞ ⎡1 1 ⎛ t ⎞ ⎤ exp ⎜ − 1 ⎟ × ⎢ ∫ exp ⎜ − 2 ⎟ dt2 ⎥ dt1 ⎝ ⎠ ⎝ τ CT ⎠ ⎦ τ τ CH 0 CH ⎣ 0 CT τ CH = τ CH + τ CT

CT PCH =

∫τ

τ CT = τ CH

coup RPES iso NEXAFS

I

I

iso RPES

/I

coup / I NEXAFS coup coup − I RPES / I NEXAFS

(5)

The core-hole lifetime τCH has been measured to be 6 fs for C 1s, 6 fs for N 1s, 4 fs for O 1s and 0.5 fs for S 2s.[21] For a more detailed explanation and derivation of the equations described above, the reader is referred to the excellent review by Brühwiler et al.[19] and references therein.

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(1)

In Equation (1) the upper limit of the integral for the corehole decay is taken to be infinity reflecting the fact that the system eventually decays with no core-holes left when measCT uring a spectrum. On the other hand, the probability PCH is a measure of the normal Auger (non-resonant) fractional intensity with respect to the total spectral intensity (resonant plus non-resonant). Hence we have CT PCH =

To calibrate the intensity, the participator intensity (IRPES) is normalized to that of the corresponding near-edge x-ray absorption fine structure (NEXAFS) resonance (INEXAFS), that is I = IRPES/INEXAFS. The charge transfer time can then be obtained from the relation

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photoemission spectrum. The BE of the resonant photoemission peak is constant as the photon energy sweeps across the absorption threshold. In contrast, if the molecular orbitals are strongly coupled to the substrate, the excited electrons has a high probability of being transferred to the substrate conduction band (Figure 1f) before the core-hole decay occurs. As a result, the autoionization decay process is greatly quenched and core-holes decay via normal Auger (non-resonant) process (Figure 1g). Assuming the charge transfer process and core-hole decay process are two independent decay channels both governed by exponential decay laws with characteristic times τCT and τCH, respectively,[19] the conditional probability (PCT CH) of the event in which charge transfer occurs before core-hole decay is given by

I Aug τ CH = τ CH + τ CT I Aug + I res

(2)

for which IAug represents the intensity of the normal Auger component, and Ires the intensity of the autoionziation component. The charge transfer time is then obtained from the relation

τ CT = τ CH

CT 1 − PCH CT PCH

(3)

In practice, the deconvolution of the normal Auger spectral component and the autoionziation component is usually achieved by fitting the measured CHC spectrum to linear combinations of the normal Auger spectrum taken at non-resonant photon energy and the “pure” resonant spectrum (i.e., in isolated system) with appropriate weight factors. An example of the application of Equation (3) is shown in section 3.3. However, when the normal Auger and autoionziation spectral features are strongly overlapping, it is usually difficult to resolve the spectrum. In this case, one can approximately take the intensity of the participator decay channel measured Iiso for isolated system (e.g., as in multilayer) as proportional to the total spectral intensity, whereas a decrease in the intensity of that particular participator feature measured in the coupled system Iiso – Icoup corresponds to the charge transfer process, and hence the normal Auger decay. Hence the relationship in Equation (2) is rewritten as

τ CH I iso − I coup = τ CH + τ CT I iso


(4)

3. Charge Transfer Dynamics at Organic/Electrode Interfaces 3.1. Charge Transfer Timescale Between Organic Semiconductors and Metal Substrates Metals, such as Au and Al, are widely used as electrode materials in organic electronic devices. In general, the interfacial electron transfer rate largely depends on two factors: energetic alignment (of excited states) and electronic coupling strength,[22] which are in turn modulated at the molecular scale by the orientation and supramolecular organization of organic semiconducting molecules at interfaces.[23] A necessary condition for charge delocalization is that it is energetically favourable for charge transfer to occur between frontier unoccupied molecular orbitals (i.e., LUMO, LUMO+1, LUMO+2 etc.) and Fermi level (EF) or conduction band edge of the electrode materials. Note that the energy positions of these unoccupied orbitals are usually shifted towards EF or even below EF in the excited state due to the Coulomb interaction between the core-hole (c.f. Figure 1c) and the photo-excited electron (i.e., core-hole excitonic effect).[24] For individual LUMO resonances which lie below EF of the metal substrate, the transfer of photo-excited electrons to the substrate is energetically forbidden. In this case, charge transfer from higher lying orbitals (e.g., LUMO+1, LUMO+2) that overlap energetically with the substrate conduction band is studied using CHC, and the corresponding charge transfer dynamics is assumed to be similar to that of the LUMO. On the other hand, for those LUMO resonances lie above EF, the transfer of photo-excited electrons to the electrode is energetically favourable but still subject to the electronic coupling strength. It should be noted that in a real organic device, the LUMO would usually lie above the EF or conduction band edge of electrodes even with the presence of charge carriers. An example to illustrate this phenomenon is 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Au(111) system.[25] As evident from angular dependent NEXAFS spectra,[26] PTCDA molecules adopt a near lying down configuration for both monolayer and multilayer regions with high degree of orientational order. Figure 2a and 2b show the CHC spectroscopy spectra for monolayer and multilayer PTCDA on Au(111) across the C 1s → π* absorption threshold, respectively.



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Figure 2. CHC spectroscopy spectra for monolayer (a) and multilayer (b) PTCDA on Au(111). The bottom spectra are corresponding valence band spectra measured with photon energy of 60 eV and the spectra on the left side is their respective NEXAFS spectrum. (c) An illustration of interfacial charge transfer for PTCDA molecule physisorbed on Au(111) with lying down configuration. (d) Calculated LUMO and LUMO+1 orbital of BDA and an illustration of charge transfer from BDA molecule to Au(111) with both lying down or tilted configuration. Reproduced with permission.[25] Copyright 2011, AIP Publishing LLC. Reproduced with permission.[30] Copyright 2013, American Chemical Society.

The photon energies of resonances correspond well with the four absorption features in the NEXAFS spectra. Lower BE resonant features (denoted by dashed lines) are mainly associated with the resonant enhancement of individual HOMO and HOMO-1 ∼ HOMO-3, and they are relatively discrete in energy. On the other hand, broad resonant structures at higher BE (above 8 eV) are mostly contributed by the resonant Auger and normal Auger processes. It is worth noting that the resonant molecular orbital-derived valence band features show dissimilar photon energy dependence, for example, (i) the HOMO and HOMO-1 derived features resonate at photon energies ranging from 283.4 eV to 286.6 eV associated with the Cperylene 1s → LUMO and LUMO+1∼LUMO+3 transitions, whereas they nearly vanish at higher photon energies corresponding to Canhydride 1s → LUMO and LUMO+1∼LUMO+3 transitions; (ii) the HOMO-2 related resonance can be observed for all four π* absorption peaks in the C K-edge NEXAFS spectrum. Similar to the observation of monolayer NTCDA on Ag(111),[27] it could be related to the photon energy dependent enhancement of valence states: Resonant enhancement of a valence orbital derived feature is most evident for photon energies corresponding to excitations to the unoccupied orbitals (LUMOs) which are spatially overlapped with the specific valence orbitals (HOMOs).[28] The electron transfer from the excited LUMO states of the molecules to the conduction band of substrate is energetically unfavorable because the measured LUMO lies below the substrate EF. Consequently, only higher lying empty molecular orbitals of LUMO+1∼LUMO+3 can participate in the interfacial charge transfer. The relative integrated participator signal between 0 eV and 4 eV in BE excluding Auger-type signals,

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e.g., spectator decay and normal Auger decay, compared with iso NEXAFS intensity at different resonant energies are I L+1~L+3 coup (Cperylene) = 1.36 ± 0.01, I L+1~L+3 (Cperylene) = 1.37 ± 0.02; and coup iso I L+1~L+3 (Canhydride) = 0.31 ± 0.02, I L+1~L+3 (Canhydride) = 0.31 ± 0.04. Substituting these values into Equation (5), the charge transfer timescale of τL+1∼L+3 > 60 fs is obtained at both Cperylene sites and Canhydride sites as shown in Figure 2c, that is, the charge transfer time is beyond the highest limit (∼10×core-hole lifetime) of the CHC technique due to the comparable Icoup and Iiso values, within experimental error. The slow charge transfer dynamics for monolayer PTCDA on gold mainly originates from weak electronic coupling and predominant van der Waals-type interactions at the PTCDA/Au(111) interface, despite the flat lying molecular geometry which could potentially maximize the d-π interactions at the interface.[29] In contrast, if strong molecule-metal interactions prevail at the interface, such as donor-acceptor bonds and particularly covalent bonds, significantly faster charge transfer across the interface will occur. This is the case of 1,4-benzenediamine (BDA) molecules bound on gold though N-Au donor-acceptor bonds in the study by Kladnik et al.[30] The charge transfer is energetically favourable at LUMO and LUMO+1 orbitals because they both lie above the EF of substrate even in their excited states. Since nitrogen atoms do not contribute any weight to the LUMO orbital (c.f. Figure 2d), N 1s electrons can only be photo-excited to LUMO+1 orbitals. Charge delocalization from nitrogen sites (using N 1s to LUMO+1 resonant energy) to Au across N-Au donor-acceptor bond is determined to be τL+1(N) < 0.5 fs for the flat lying geometry as shown in Figure 2d, that is, the charge transfer timescale is beyond the lowest limit (∼0.1 × core-hole lifetime) of the CHC technique due to




3.2. Charge Transfer Timescale between Organic Dyes and Metal Oxide Substrates Since the discovery of the Grätzel cell or DSSC, TiO2 has been considered as one of the most promising photoanode materials due to its excellent photoelectrochemical property, high corrosion resistance, high stability, low cost, high efficiency and environmental friendliness. The (4,4′-dicarboxy2,2′-bipyridine)2Ru(NCS)2, commonly known as N3 dye, and its derivatives are the most interesting sensitized dyes using in DSSCs because they exhibit (i) efficient absorption from the visible spectrum, (ii) higher stability in the oxidized state, (iii) record conversion efficiency higher than 11%, and (iv) structural tunability. It is well known that the exciton dissociation through ultrafast electron transfer from the photo-excited state of the organic dye into the conduction band of large bandgap semiconductor TiO2, is a critical process that competes against various loss processes, e.g., charge recombination at the interface, charge redistribution, and intramolecular


thermalization of excited states. CHC spectroscopy is an ideal tool to study this exciton dissociation process. The charge transfer dynamics at model interfaces formed between biisonicotinic acid (4,4′-dicarboxy- 2,2′-bipyridine), which is the ligand of N3 dye, and rutile TiO2(110) was first investigated by Schnadt et al. in 2002 using CHC spectroscopy,[13] and an ultrafast charge transfer timescale of τ < 2.5 fs was estimated. It was later found that the covalent Ti-O bond formation through deprotonation of carboxylic acid groups is key to the ultrafast charge transfer, which allows direct electronic coupling of the unoccupied levels to the substrate conduction band even when the bond sites are reduced.[33] Following these initial studies, ligands related to N3 dye and other types of organic dyes including ruthenium complexes (N3 and its derivatives) and metal free organic dye have been examined using CHC spectroscopy.[34] In general, the LUMO of those organic dyes lies energetically within the band gap of TiO2 in the excited states, which prevents the charge transfer because the transition is energetically forbidden. Therefore, all studies of charge transfer focus on higher unoccupied orbitals. It was found that charge transfer from N3 dye specific unoccupied orbitals, which is located on bis-isonicotinic acid ligands, to substrate occurs in less than 16 fs considering the similar covalent Ti-O bonding geomery, whereas charge transfer from unoccupied orbitals localized at the central Ru atom or thiocyanate ligands is longer due to weaker electronic coupling.[35] For N3 derivatives in which bis-isonicotinic or thiocyanate ligands were substituted, the upper limit on charge transfer times either increases from 12 fs for N3 dye to 17∼21 fs[36] or decreases to 0.9∼5.9 fs.[37] Consequently, understanding the dependence of charge transfer times on different sites or ligand substitution provides imporant insights to tuning the charge transfer dynamics at dye/TiO2 interfaces by engineering the electronic structures and binding geometries of dyes.[34e]

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complete quenching of Icoup, whereas τL+1 (N) >10 fs for tilted orientation. On the other hand, charge transfer from carbon sites (using C 1s to LUMOs transition energy) to Au is estimated to be τL (C) = 14 ± 3 fs and τL+1 (C) = 7 ± 1 fs for the flat lying orientation. The orientation and orbital (or site) dependence of the charge transfer timescale are attributed to the different electronic coupling strength between excited sites and substrate: (i) the coupling strength reduces for tilted orientation with one Au-N bond as compared to flat lying with two Au-N bonds; (ii) LUMO+1 has the orbital weight of N-atoms which can efficiently delocalize the electrons compared to the LUMO which is mainly localized on the C-atoms (c.f. Figure 2d). Not only can the orientation and orbital (or site) dependence of the charge transfer times be assessed by the CHC technique, the influence of orbital polarization, that is the spatial orientation of orbitals with the same atomic characters at the same sites, on the charge transfer process can also be evaluated. Resonances of adsorbates with different spatial orientations with respect to the substrate can be selectively excited by exploiting their polarization-dependence on the linearly-polarized incident x-rays. An example of this is the c(4×2)S/Ru(0001) system, in which the charge transfer times from the S 3p antibonding resonance perpendicular to the surface plane and that along the surface plane to the ruthenium substrate are determined to be 0.18 ± 0.07 fs and 0.84 ± 0.23 fs, respectively.[31] This difference in charge transfer times stems from the different adsorbatesubstrate orbital overlap. A similar study is reported for Au-S(CH2)2-C≡N self-assembled monolayers (SAMs), in which the π* resonances associated with the terminal cyano moieties (C≡N) have in-plane and out-of-plane components with respect to the gold substrate.[32] Consequently, by varying the incident x-ray polarization CHC spectroscopy reveals different charge transfer times from C≡N groups to the substrate for these two π* orbitals. The much faster charge transfer process (11.7 fs) from the out-of-plane orbital as compared to that from the inplane orbital (26 fs) is believed to be due to the extension of the out-of-plane π* orbital to the anchoring S atom through the alkyl backbone.

3.3. Charge Transfer Dynamics between Active Groups of the SAMs and Metal Substrates Through Molecular Backbone Functionalized molecules organized to SAMs are another key enabler for organic electronics, because they combine the advantages of low-temperature solution processing under ambient conditions, large scale fabrication, angstrom-scale control over the film thickness and order, and stability and reproducibility under electrochemical or vacuum environments. Given the great tunability of molecular structure of SAMs, devices with tuneable electronic function at molecular scale can be achieved when SAMs serve as part of the active device layer.[38] One of the main goals of SAMs-based organic and molecular electronics is to relate the charge transport properties of devices to the chemical structure of organic component and molecule-substrate interactions. This goal is difficult to be achieved by conventional I−V measurement techniques because (i) they do not provide element-specific selectivity and (ii) they inevitably increase system complexity by introducing additional top interfaces associate with the contact electrode. In a recent study carried out by Zharnikov’s group, CHC spectroscopy has been employed to study charge transport dynamics through the SAMs units to the electrodes.[32,39] A



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Figure 3. SAMs precursors (a) and N K-edge NEXAFS spectrum for NC-OPE1 on Au (b). The inset in (b) shows the calculated molecular orbitals. The CHC spectroscopy at π1* and π3* resonant energies fitting by linear combination of normal Auger spectrum (red curve) contributions of NC-OPE1 and ‘pure’ resonant spectrum (blue line) of NC-OPE3 for NC-OPE1 (c), NC-PT1 (d) and NC-BP0 (e), respectively. Reproduced with permission.[39b] Copyright 2011, American Physical Society.

specific tail group of cyano C≡N was introduced to the end of molecular backbone of SAMs molecules studied. Using the N atom as the excitation site can then allow the charge transfer path through the molecular backbone and across the headgroup-substrate anchor to be unambiguously examined. By careful designing the SAMs molecular structures, they have revealed the dependence of the charge transfer dynamics on the character of the molecular orbitals which mediate the charge transfer process. Hamoudi et al. used CHC spectroscopy to investigate oligo(phenylenethynylenes) (OPE) chains with different lengths.[39b] Figure 3 shows schematics of the various SAMs precursors studied and their corresponding NEXAFS and CHC spectroscopy spectra. Figure 3b shows the NEXAFS spectrum of NC-OPE1 molecules, which shows various resonances corresponding to transitions to the unoccupied π1* and π3* states. As shown in the inset of Figure 3b, π1* is delocalized over the entire benzonitrile moiety, whereas π3* is localized on the nitrile group. For NC-OPE2 and NC-OPE3 with longer OPE chains, the π1* orbitals lie below the EF of metal substrate. The charge transfer from π1* orbitals to substrate is therefore energetically forbidden. It corresponds well with the observation that no perceptible normal Auger signals is observed in the resonant spectra at π1* resonant energy. Similarly, “pure” resonant enhancement without clear Auger contribution is observed at π3* resonance (c.f. blue curve in Figure 3e), indicating that the charge transfer timescale is longer than the core-hole lifetime of N 1s, and beyond the range accessible by CHC spectroscopy.

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The situation is dramatically different for NC-OPE1 molecules with shorter chain lengths. As shown in Figure 3c, the resonant spectrum at π1* resonance can be best fitted by a linear comCT bination ∼41% NC-OPE1 normal Auger spectrum (PCH = 0.41) (red curve) and ∼59% NC-OPE3 “pure” resonant spectrum (blue curve) due to combined resonant Auger decay and resonant photoemission decay. At π3* resonance, the linear combination is ∼17% normal Auger spectrum and ∼83% resonant spectrum. As normal Auger signals indirectly represent the possibility of charge transfer, larger normal Auger weight indicates faster charge transfer process. The charge transfer times are then estimated to be τπ1(NC-OPE1) = 9 ± 3 fs and τπ3 (NC-OPE1) = 31.5 ± 4.5 fs using Equation (3). It is worth noting that the charge transfer dynamics exhibit orbital dependence so that the π1* orbital is much more efficient than the π3* orbital. With insertion of a single CH2 unit in the molecular backbone (NC-PT1), the charge transfer time increases significantly to τπ1 (NC-PT1) = 19.2 ± 5 fs and τπ3 (NC-PT1) = 60 ± 10 fs, indicating its strong effect on charge transfer. With insertion of a single phenyl unit (NC-BP0), the charge transfer time increases further to τπ1 (NC-BP0) = 29 ± 6 fs and τπ3 (NC-BP0) >> 60 fs due to the quench of normal Auger signal as shown in Figure 3e. The increasing of charge transfer timescale is attributed to further decoupling between excitation sites and substrates. Note that a series of SAMs with repeating units can be designed to study the length (l) dependence of charge transfer timescale. The resistance (R) of the molecular junction and molecular contact resistance (R0) could be describe by




molecular devices by selectively injecting charge carriers into faster molecular orbital channels. The successful investigations of charge transfer dynamics by CHC spectroscopy in SAMs systems illustrate its usefulness in understanding the dependence of the charge transport through the backbone of SAMs on the nature of the molecular orbitals. Therefore, CHC spectroscopy has the potential of becoming a powerful tool to investigate the effects of different functional groups on the conductance of SAMs, providing valuable input for the rational design of the molecular structures of SAMs.

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the equation R = R0 exp(+βl). Similarly, the charge transfer mechanisms can be described by the formula τCT = τS-metal exp(+βl), where τS-Metal represents the charge transfer timescale through the S-Au bond. The value of the attenuation factor (β) and τS-metal could then be derived from CHC spectroscopy without having the contact problem usually encountered in conventional I−V measurements. An example of this is the study on alkyl backbone series (NC-Cn) of n = 2, 3 and 4 by Kao et al.,[39a] the charge transfer from C≡N tail to substrate was estimated to be on the timescale of τπ* (NC-C2) = 15 ± 4 fs, τπ* (NC-C3) = 35 ± 9 fs and τπ* (NC-C4) = 100 ± 26 fs, respectively.[39a,c] For saturated chains backbone, the charge transfer timescale increases exponentially as a function of number of CH2 units with β = 0.72 Å−1 (0.93 per CH2) and τS-metal = 2.3 fs.[39a] This derived β value is comparable to the static attenuation factor of 0.7 ∼ 0.9 Å−1 assuming charge tunnelling along the chain. By considering NC-OPE1 and NC-BP0 as part of the oligophenyl (OPh) series, the charge transfer from π1* orbital across thiolate anchor τS-Au was estimated to be 2.8 fs and βOPh(π1*) is 0.27 Å−1 (1.17 per phenyl ring). Taking NC-OPE1 and NC-PT1 as part of the OPh series, the formula τCT = τS-metal exp(+βAlA) exp(+βOPhlOPh), was used by Hamoudi et al.[39b] with βA and lA representing the length and attenuation factor respectively of the aliphatic. Finally, βOPh is estimated to be 0.29 Å−1(1.25 per phenyl ring) and 0.55 Å−1(2.34 per phenyl ring) for the π1* and π3* resonance, respectively. For the OPh chain, the βOPh (π3*) of 0.55 Å−1 can be compared to the β factor of 0.41∼0.7 Å−1 provided by static I−V measurements, whereas βOPh (π1*) of 0.27 Å−1 or 0.29 Å−1 deviates greatly. The fact that the static tunneling in previous I−V measurements did not occur over the most suitable molecular orbital (e.g., π1*) suggest that there is still great space to improve the charge transport efficiency in SAMs-based

3.4. Charge Transfer Dynamics Through-Space within π-Coupled Molecule Electron transport in π-conjugated molecules can be categorized as through-bonds (intramolecule) or through-space (intermolecule). Charge transfer across the backbone of SAMs discussed above is an example of through-bond charge transport. Electron transport though-space is an important property in bulk (multilayer stacks) devices involving π-stacking conjugated molecules such as OPVCs and OLEDs, and largely determines the charge carrier mobility in organic semiconductor materials. CHC spectroscopy can quantify the though space charge transfer dynamics in specially designed molecules with π-stacked geometry. Batra et al. studied the through-space charge transfer dynamics in π-coupled molecular systems.[40] [2,2]Paracyclophane (22PCP) and [4,4]paracyclophane (44PCP), which can be represented as two benzene rings stacked together with aliphatic carbon chains as supports, provide an ideal platform to study the effect of inter-ring coupling. Figure 4 shows the CHC spectroscopy spectra and schematics of 44PCP and 22PCP on Au(111). It was found that 22PCP and 44PCP molecules adopt

Figure 4. CHC spectroscopy spectra for multilayer (a) and monolayer (b) of 44PCP on Au(111). (c) A schematic of charge transfer time estimated by CHC spectroscopy. The corresponding panels for 22PCP are shown in (d), (e) and (f). Reproduced with permission.[40] Copyright 2012, Macmillan Publishers Ltd: Nature Communications.




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a similar adsorption geometry on the Au surface with average tilt angles of 47 ± 5° and 45 ± 5° between aromatic rings and Au surface as shown in Figure 4c and Figure 4f, respectively. After careful normalization, the integrated intensities for both monolayer and multilayer are used to calculate charge transfer between aromatic ring and Au substrate. In general, the intensity for the monolayer (Figure 4b for 44PCP and 4e for 22PCP) is reduced compared with that of the multilayer (Figure 4a for 44PCP and 4d for 22PCP), indicating that electrons are transferred to the substrate within the core-hole lifetime. The average charge transfer time for the monolayer is estimated to be τLUMO(44PCP) = 6.0 ± 0.6 fs and τLUMO(22PCP) = 1.4 ± 0.5 fs. This average charge transfer time is contributed by two parts: (i) charge transfer between bottom rings (τbottom) and the substrate, and (ii) charge transfer between top rings (τtop) and the substrate. Assuming the charge transfer times from the bottom rings of both 22PCP and 44PCP monolayers are equivalent, then the charge transfer between top rings and substrate is mainly modulated by electronic coupling between the rings. The charge transfer time from bottom rings is determined to be τbottom = 0.7 ± 0.3 fs, whereas that from the top rings to substrate though inter-ring space is τtop(44PCP) ≥ 50 fs and τtop(22PCP) = 2.3 ± 0.6 fs. It is observed that an increase of the π−π coupling distance by insertion of a single CH2 unit significantly slower the through-space charge transfer rate by ∼20 times. In addition to systematically varying the inter-ring distance in the π-stacked system, other types of π-stacked molecules in which either the benzene dimer is made to have variable parallel displacement or to have variable tilt angles between the benzene planes could be designed and examined by CHC spectroscopy, thus helping us to understand the through-space charge transfer dynamics in more complex but more technical relevant molecular assemblies. Quantitative understanding of the through-space charge transfer dynamics in extended π-conjugated system by CHC spectroscopy can be directly translated to the exciton dynamics which are fundamental to the optimization of OPVCs and OLEDs.

4. Conclusion In summary, by showing several examples of molecule-substrate systems, we have shown that the study of charge transfer dynamics with CHC spectroscopy finds many excellent applications at organic/electrode interfaces, offering important information, such as molecule-to-substrate charge transfer timescale, charge transfer timescale through-bond or throughspace, attenuation factors, and electron delocalization between organic donor and acceptor.[41] The chemical element and/or chemical environment (or orbitals) selectivity which is inherent to the CHC technique could help us establish the complex relationship between molecular structures, supramolecular packing, molecule-substrate interactions, molecular orbital symmetry and device performance, thus enabling the next-generation organic electronic devices through rational design. With the development of higher spatial resolution of synchrotron radiation techniques and/or electron analyzer, charge transfer dynamics could be coupled with spatial resolution. For

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example, when the spatial resolution is comparable with the typical channel length of the OTFT devices or donar/acceptor domain size in bulk heterojunction organic solar cells, insitu characterization of charge transfer dynamics in operating device could be realized.

Acknowledgements Dr. X.-Y. GAO is grateful for the support of the National Natural Science Foundation of China (Grant No. 11175239). Received: October 31, 2013 Revised: February 28, 2014 Published online:

[1] J. Li, Y. Zhao, H. S. Tan, Y. Guo, C.-A. Di, G. Yu, Y. Liu, M. Lin, S. H. Lim, Y. Zhou, H. Su, B. S. Ong, Sci. Rep. 2012, 2, 754. [2] Y. Lin, H. Fan, Y. Li, X. Zhan, Adv. Mater. 2012, 24, 3087. [3] a) A. W. Hains, Z. Liang, M. A. Woodhouse, B. A. Gregg, Chem. Rev. 2010, 110, 6689; b) R. A. J. Janssen, J. Nelson, Adv. Mater. 2013, 25, 1847. [4] S. Reineke, M. Thomschke, B. Lüssem, K. Leo, Rev. Mod. Phys. 2013, 85, 1245. [5] a) Y. Guo, G. Yu, Y. Liu, Adv. Mater. 2010, 22, 4427; b) L. Torsi, M. Magliulo, K. Manoli, G. Palazzo, Chem. Soc. Rev. 2013, 42, 8612; c) C. B. Nielsen, M. Turbiez, I. McCulloch, Adv. Mater. 2013, 25, 1859. [6] a) B. E. Hardin, H. J. Snaith, M. D. McGehee, Nat. Photon. 2012, 6, 162; b) P. R. F. Barnes, K. Miettunen, X. Li, A. Y. Anderson, T. Bessho, M. Gratzel, B. C. O’Regan, Adv. Mater. 2013, 25, 1881. [7] P. Heremans, G. H. Gelinck, R. Müller, K.-J. Baeg, D.-Y. Kim, Y.-Y. Noh, Chem. Mater. 2010, 23, 341. [8] P. Lin, F. Yan, Adv. Mater. 2012, 24, 34. [9] S. Sanvito, Chem. Soc. Rev. 2011, 40, 3336. [10] J. Meyer, S. Hamwi, M. Kröger, W. Kowalsky, T. Riedl, A. Kahn, Adv. Mater. 2012, 24, 5408. [11] J. Hwang, A. Wan, A. Kahn, Mater. Sci. Eng. R-Rep. 2009, 64, 1. [12] a) X. Y. Zhu, J. Phys. Chem. B 2004, 108, 8778; b) X. Y. Zhu, Surf. Sci. Rep. 2004, 56, 1. [13] J. Schnadt, P. A. Bruhwiler, L. Patthey, J. N. O’Shea, S. Sodergren, M. Odelius, R. Ahuja, O. Karis, M. Bassler, P. Persson, H. Siegbahn, S. Lunell, N. Martensson, Nature 2002, 418, 620. [14] A. Föhlisch, P. Feulner, F. Hennies, A. Fink, D. Menzel, D. SánchezPortal, P. M. Echenique, W. Wurth, Nature 2005, 436, 373. [15] D. Menzel, Chem. Soc. Rev. 2008, 37, 2212. [16] a) P. Emma, R. Akre, J. Arthur, R. Bionta, C. Bostedt, J. Bozek, A. Brachmann, P. Bucksbaum, R. Coffee, F. J. Decker, Y. Ding, D. Dowell, S. Edstrom, A. Fisher, J. Frisch, S. Gilevich, J. Hastings, G. Hays, P. Hering, Z. Huang, R. Iverson, H. Loos, M. Messerschmidt, A. Miahnahri, S. Moeller, H. D. Nuhn, G. Pile, D. Ratner, J. Rzepiela, D. Schultz, T. Smith, P. Stefan, H. Tompkins, J. Turner, J. Welch, W. White, J. Wu, G. Yocky, J. Galayda, Nat. Photon. 2010, 4, 641; b) T. Ishikawa, H. Aoyagi, T. Asaka, Y. Asano, N. Azumi, T. Bizen, H. Ego, K. Fukami, T. Fukui, Y. Furukawa, S. Goto, H. Hanaki, T. Hara, T. Hasegawa, T. Hatsui, A. Higashiya, T. Hirono, N. Hosoda, M. Ishii, T. Inagaki, Y. Inubushi, T. Itoga, Y. Joti, M. Kago, T. Kameshima, H. Kimura, Y. Kirihara, A. Kiyomichi, T. Kobayashi, C. Kondo, T. Kudo, H. Maesaka, X. M. Marechal, T. Masuda, S. Matsubara, T. Matsumoto, T. Matsushita, S. Matsui, M. Nagasono, N. Nariyama, H. Ohashi, T. Ohata, T. Ohshima, S. Ono, Y. Otake, C. Saji, T. Sakurai, T. Sato, K. Sawada, T. Seike, K. Shirasawa, T. Sugimoto, S. Suzuki, S. Takahashi, H. Takebe,




[18]

[19] [20]

[21]

[22] [23]

[24]

[25]

[26] [27] [28] [29] [30]


[31] M. Deppe, A. Föhlisch, F. Hennies, M. Nagasono, M. Beye, D. Sánchez-Portal, P. M. Echenique, W. Wurth, J. Chem. Phys. 2007, 127, 174708. [32] F. Blobner, P. B. Coto, F. Allegretti, M. Bockstedte, O. Rubio-Pons, H. Wang, D. L. Allara, M. Zharnikov, M. Thoss, P. Feulner, J. Phys. Chem. Lett. 2012, 3, 436. [33] J. Schnadt, J. N. O’Shea, L. Patthey, L. Kjeldgaard, J. Åhlund, K. Nilson, J. Schiessling, J. Krempaský, M. Shi, O. Karis, C. Glover, H. Siegbahn, N. Mårtensson, P. A. Brühwiler, J. Chem. Phys. 2003, 119, 12462. [34] a) K. Westermark, H. Rensmo, J. Schnadt, P. Persson, S. Södergren, P. A. Brühwiler, S. Lunell, H. Siegbahn, Chem. Phys. 2002, 285, 167; b) E. M. J. Johansson, M. Hedlund, M. Odelius, H. Siegbahn, H. Rensmo, J. Chem. Phys. 2007, 126, 244303; c) J. B. Taylor, L. C. Mayor, J. C. Swarbrick, J. N. O’Shea, J. Schnadt, J. Phys. Chem. C 2007, 111, 16646; d) S. Yu, S. Ahmadi, M. Zuleta, H. Tian, K. Schulte, A. Pietzsch, F. Hennies, J. Weissenrieder, X. Yang, M. Göthelid, J. Chem. Phys. 2010, 133, 224704; e) L. Cao, Y.-Z. Wang, J.-Q. Zhong, Y.-Y. Han, W.-H. Zhang, X.-J. Yu, F.-Q. Xu, D.-C. Qi, A. T. S. Wee, J. Phys. Chem. C 2014, 118, 4160. [35] L. C. Mayor, J. Ben Taylor, G. Magnano, A. Rienzo, C. J. Satterley, J. N. O’Shea, J. Schnadt, J. Chem. Phys. 2008, 129, 114701. [36] M. Weston, A. J. Britton, J. N. O’Shea, J. Chem. Phys. 2011, 134, 054705. [37] M. Weston, K. Handrup, T. J. Reade, N. R. Champness, J. N. O’Shea, J. Chem. Phys. 2012, 137, 224706. [38] a) A. Salomon, D. Cahen, S. Lindsay, J. Tomfohr, V. B. Engelkes, C. D. Frisbie, Adv. Mater. 2003, 15, 1881; b) J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo, G. M. Whitesides, Chem. Rev. 2005, 105, 1103; c) R. L. McCreery, A. J. Bergren, Adv. Mater. 2009, 21, 4303; d) S. A. DiBenedetto, A. Facchetti, M. A. Ratner, T. J. Marks, Adv. Mater. 2009, 21, 1407; e) M. Halik, A. Hirsch, Adv. Mater. 2011, 23, 2689. [39] a) P. Kao, S. Neppl, P. Feulner, D. L. Allara, M. Zharnikov, J. Phys. Chem. C 2010, 114, 13766; b) H. Hamoudi, S. Neppl, P. Kao, B. Schüpbach, P. Feulner, A. Terfort, D. Allara, M. Zharnikov, Phys. Rev. Lett. 2011, 107, 027801; c) S. Neppl, U. Bauer, D. Menzel, P. Feulner, A. Shaporenko, M. Zharnikov, P. Kao, D. L. Allara, Chem. Phys. Lett. 2007, 447, 227. [40] A. Batra, G. Kladnik, H. Vázquez, J. S. Meisner, L. Floreano, C. Nuckolls, D. Cvetko, A. Morgante, L. Venkataraman, Nat. Commun. 2012, 3, 1086. [41] P. Vilmercati, C. Castellarin-Cudia, R. Gebauer, P. Ghosh, S. Lizzit, L. Petaccia, C. Cepek, R. Larciprete, A. Verdini, L. Floreano, A. Morgante, A. Goldoni, J. Am. Chem. Soc. 2008, 131, 644.



RESEARCH NEWS

[17]

K. Takeshita, K. Tamasaku, H. Tanaka, R. Tanaka, T. Tanaka, T. Togashi, K. Togawa, A. Tokuhisa, H. Tomizawa, K. Tono, S. Wu, M. Yabashi, M. Yamaga, A. Yamashita, K. Yanagida, C. Zhang, T. Shintake, H. Kitamura, N. Kumagai, Nat. Photon. 2012, 6, 540. L. Young, E. P. Kanter, B. Krässig, Y. Li, A. M. March, S. T. Pratt, R. Santra, S. H. Southworth, N. Rohringer, L. F. DiMauro, G. Doumy, C. A. Roedig, N. Berrah, L. Fang, M. Hoener, P. H. Bucksbaum, J. P. Cryan, S. Ghimire, J. M. Glownia, D. A. Reis, J. D. Bozek, C. Bostedt, M. Messerschmidt, Nature 2010, 466, 56. A. Pietzsch, A. Föhlisch, M. Beye, M. Deppe, F. Hennies, M. Nagasono, E. Suljoti, W. Wurth, C. Gahl, K. Dobrich, A. Melnikov, New J. Phys. 2008, 10, 033004. P. A. Brühwiler, O. Karis, N. Mårtensson, Rev. Mod. Phys. 2002, 74, 703. a) L. Wang, W. Chen, A. T. S. Wee, Surf. Sci. Rep. 2008, 63, 465; b) P. Vilmercati, D. Cvetko, A. Cossaro, A. Morgante, Surf. Sci. 2009, 603, 1542; c) R. Friedlein, S. Braun, M. P. de Jong, W. Osikowicz, M. Fahlman, W. R. Salaneck, J. Electron Spectrosc. Relat. Phenom. 2011, 183, 101. a) B. Kempgens, A. Kivimäki, M. Neeb, H. M. Köppe, A. M. Bradshaw, J. Feldhaus, J. Phys. B: At., Mol. Opt. Phys. 1996, 29, 5389; b) M. Coville, T. D. Thomas, Phys. Rev. A 1991, 43, 6053; c) M. O. Krause, J. H. Oliver, J. Phys. Chem. Ref. Data 1979, 8, 329. H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 1999, 11, 605. a) W. Chen, D.-C. Qi, H. Huang, X. Gao, A. T. S. Wee, Adv. Funct. Mater. 2011, 21, 410; b) W. Chen, D. C. Qi, Y. L. Huang, H. Huang, Y. Z. Wang, S. Chen, X. Y. Gao, A. T. S. Wee, J. Phys. Chem. C 2009, 113, 12832; c) S. Braun, W. R. Salaneck, M. Fahlman, Adv. Mater. 2009, 21, 1450. a) J. Schnadt, J. Schiessling, P. A. Brühwiler, Chem. Phys. 2005, 312, 39; b) A. J. Britton, M. Weston, J. B. Taylor, A. Rienzo, L. C. Mayor, J. N. O’Shea, J. Chem. Phys. 2011, 135, 164702. L. Cao, Y. Z. Wang, T. X. Chen, W. H. Zhang, X. J. Yu, K. Ibrahim, J. O. Wang, H. J. Qian, F. Q. Xu, D. C. Qi, A. T. S. Wee, J. Chem. Phys. 2011, 135, 174701. J. Stöhr, NEXAFS Spectroscopy Springer-Verlag, Berlin, 1992. A. Bendounan, F. Forster, A. Schöll, D. Batchelor, J. Ziroff, E. Umbach, F. Reinert, Surf. Sci. 2007, 601, 4013. J. Kikuma, B. P. Tonner, J. Electron Spectrosc. Relat. Phenom. 1996, 82, 41. S. Duhm, A. Gerlach, I. Salzmann, B. Bröker, R. L. Johnson, F. Schreiber, N. Koch, Org. Electron. 2008, 9, 111. G. Kladnik, D. Cvetko, A. Batra, M. Dell’Angela, A. Cossaro, M. Kamenetska, L. Venkataraman, A. Morgante, J. Phys. Chem. C 2013, 117, 16477.

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Charge Transfer Dynamics at Dye-Sensitized ZnO and TiO2 Interfaces Studied by Ultrafast XUV Photoelectron Spectroscopy.

Adsorption of firefly luciferase at interfaces studied by total internal reflection fluorescence spectroscopy.

In-situ Multimodal Imaging and Spectroscopy of Mg Electrodeposition at Electrode-Electrolyte Interfaces.

Orbital dependent ultrafast charge transfer dynamics of ferrocenyl-functionalized SAMs on gold studied by core-hole clock spectroscopy.

Complex atrial arrhythmias studied by suction electrode technique.

Biocompatibility considerations at stimulating electrode interfaces.

(-)-R-mevalonolactone studied by ROA and SERS spectroscopy.

Metallization of warm dense SiO(2) studied by XANES spectroscopy.

Kinetics of carboxymyoglobin and oxymyoglobin studied by picosecond spectroscopy.

Ionic mobility in DNA films studied by dielectric spectroscopy.

RNA refolding studied by light-coupled NMR spectroscopy.

Electrohydrodynamic removal of non-specific colloidal adsorption at electrode interfaces.

Microfabricated nerve-electrode interfaces in neural prosthetics and neural engineering.

Integrated circuits and electrode interfaces for noninvasive physiological monitoring.

Electrooxidation of morin hydrate at a Pt electrode studied by cyclic voltammetry.

Triplet state quenching of phenosafranine dye by indolic compounds studied by transient absorption spectroscopy.

Probing the electronic and spintronic properties of buried interfaces by extremely low energy photoemission spectroscopy.

Liquid-solid interfaces: structure and dynamics from spectroscopy and simulations.

molecule interfaces by in-device electrical spectroscopy.

Biomolecular structure at solid-liquid interfaces as revealed by nonlinear optical spectroscopy.

Interactions in affinity partition studied using fluorescence spectroscopy.

Biosilica-Entrapped Enzymes Studied by Using Dynamic Nuclear-Polarization-Enhanced High-Field NMR Spectroscopy.

Magnetic properties of nano-scale hematite, α-Fe2O3, studied by time-of-flight inelastic neutron spectroscopy.

Valence band splitting in wurtzite InGaAs nanoneedles studied by photoluminescence excitation spectroscopy.