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A hitherto unknown butadiyne-linked BODIPY dimer (4) has been synthesized under the conventional Glaser coupling reaction conditions using the ethynyl-substituted pyrrolyl-BODIPY (2) prepared by gold-catalyzed C-H alkynylation. Due to the [Formula: see text]-extended structure of the BODIPY chromophore in 4, a characteristic broad absorption band in the near-infrared (NIR) region has emerged. Upon photoexcitation, the environment-dependent fluorescence spectral features were observed. The fluorescent response varied with solvents, viscosity, and temperature, which was found to originate from the conformational changes between the coplanar and twisted conformers in the otherwise rigid butadiyne-bridged dimer 4. Therefore, the BODIPY dimer 4 would be a potential NIR fluorescence material for use in the probes responding to the distinct viscous environment in biological tissues.

The synthesis of organometallic complexes of modified 26π-conjugated hexaphyrins with absorption and emission capabilities in the third near-infrared region (NIR-III) is described. Symmetry alteration of the frontier molecular orbitals (MOs) of bis-PdII and bis-PtII complexes of hexaphyrin via N-confusion modification led to substantial metal dπ -pπ interactions. This MO mixing, in turn, resulted in a significantly narrower HOMO-LUMO energy gap. A remarkable long-wavelength shift of the lowest S0 →S1 absorption beyond 1700 nm was achieved with the bis-PtII complex, t-Pt2 -3. The emergence of photoacoustic (PA) signals maximized at 1700 nm makes t-Pt2 -3 potentially useful as a NIR-III PA contrast agent. The rigid bis-PdII complexes, t-Pd2 -3 and c-Pd2 -3, are rare examples of NIR emitters beyond 1500 nm. The current study provides new insight into the design of stable, expanded porphyrinic dyes possessing NIR-III-emissive and photoacoustic-response capabilities.

An expanded metalloporphyrin-based "black dye" Au-Pd oxohexaphyrin (AuPd-1) with absorption capabilities across the visible-to-near-infrared (NIR) range was synthesized. This black dye, AuPd-1, possessed efficient light-harvesting and photostable capabilities, which were indicative of superior photothermal (PT) conversion abilities. Encapsulation of AuPd-1 with a micellar nanocapsule resulted in a compound that demonstrated intense photoacoustic (PA) properties in the NIR region in water. This finding indicated how metal (d)-ligand (π) molecular orbital interactions in metalloporphyrins could aid in the design of visible-to-NIR light-harvesting black dyes for PT and PA applications.

Hybrid metal-organic frameworks (MOFs) with core/shell-like hierarchical structure comprised of zirconium metal and porphyrin (e.g., TPP) and its isomer, N-confused porphyrin (NCP), were synthesized through a seed-mediated reaction. The hierarchical structures of hybrid MOFs were characterized by the microscopic image analyses (e.g., scanning electron microscope (SEM), energy dispersive X-ray (EDX) spectrometry, and confocal laser scanning microscope (CLSM)). Taking advantage of the intrinsic light-harvesting properties of the porphyrin dye and the N-confused isomer, changing the core/shell layer structures of hybrid MOFs allows for tuning of the visible-to-near-infrared (NIR) absorption/emission characters, excited-state energy migrations, and photosensitization capabilities. The Förster energy transfer event occurring in the bulk MOF samples by photoexcitation enabled us to control the photoinduced singlet oxygen generation through the comprehensive light-harvesting ability of these hybrid porphyrinic MOFs. Therefore, implementation of a precisely designed porphyrin "substitute" into the MOF-based materials indeed provides a new mimic of the photosynthetic pigment system and should be potentially applicable for solar-light-driven devices.

A series of rhenium(i) dipyrrinato complexes (Re1-Re8) have been prepared and characterized; their crystal structures, phosphorescence and singlet oxygen generation studies are reported. The aromatic substituents, such as thienyl, p-bromophenyl, p-fluorophenyl, m-fluorophenyl, pentaflurophenyl, N-butylcarbazole, N-phenylcarbazole, and N-butylphenothiazine, are linked to the C5 position of Re-dipyrrinates. Varying the electronic nature of the substituents from electron donating (e.g., carbazole) to electron withdrawing (e.g., pentaflurophenyl) allowed the change in the structural, electrochemical, and spectroscopic properties of these complexes. In particular, the rhenium dipyrrinates showed phosphorescence in the near IR region with sufficiently longer triplet state lifetimes (τT = 9-29 μs). Also, a large Stokes shift (Δν = 5682-6957 cm-1) was witnessed for all the rhenium dipyrrinates. Triplet emission was reflected in the efficient singlet oxygen generation yields (ΦΔ ∼ 0.75-0.98) along with the distinct photo-stability. Density functional theory (DFT) calculations revealed that the electron density is spread over the dipyrrin unit in most complexes. Rhenium dipyrrinate having a phenothiazine substituent exhibited the smallest HOMO-LUMO band gap (2.820 eV) among all Re-complexes.

Triplet photosensitizers that generate singlet oxygen efficiently are attractive for applications such as photodynamic therapy (PDT). Extending the absorption band to a near-infrared (NIR) region (700 nm≈) with reasonable photostability is one of the major demands in the rational design of such sensitizers. We herein prepared a series of mono- and bis-palladium complexes (1-Pd-H2 , 2-Pd-H2 , 1-Pd-Pd, and 2-Pd-Pd) based on modified calix[6]phyrins as photosensitizers for singlet oxygen generation. These palladium complexes showed intense absorption profiles in the visible-to-NIR region (500-750 nm) depending on the number of central metals. Upon photoirradiation in the presence of 1,5-dihydroxynaphthalene (DHN) as a substrate for reactive oxygen species, the bis-palladium complexes generated singlet oxygen with high efficiency and excellent photostability. Singlet oxygen generation was confirmed from the characteristic spectral feature of the spin trapped complex in the EPR spectrum and the intact 1 O2 emission at 1270 nm.

Organoplatinum(II) complexes of calix[4]phyrin analogues, singly N-confused calix[4]phyrin (Pt-2), and doubly N-confused calix[4]phyrin (Pt-3), were synthesized and characterized. The explicit structures of these organoplatinum(II) complexes were elucidated by single-crystal X-ray diffraction and spectroscopic studies. The introduction of N-confused pyrrole rings to the parent calix[4]phyrin scaffold was found to have profound effects on the photophysical properties, such as the bathochromic shifts of both the absorption and phosphorescence maxima. The triplet excited state properties of these platinum complexes were analyzed by DFT calculations at the B3LYP level. The organoplatinum(II) complexes derived from the deformed scaffolds can serve as potent triplet sensitizers for singlet oxygen generation under aerobic conditions.

Germanane (GeH) is a hydrogen-terminated layered crystal of germanium. We fabricated an electric double layer transistor (EDLT) on a GeH thin film on the Ge (111) substrate and investigated its electronic properties. The EDLT showed ambipolar transfer characteristics at 240 K. The sheet resistance Rsheet at null gate bias (VG = 0 V) exceeded 1 MΩ below 100 K, which indicates an insulating behavior. For VG = −2 V (hole accumulation), a gradual increase in Rsheet up to 10 kΩ was found upon decreasing temperature to 40 K, i.e., semiconducting or even weakly insulating. Remarkably, Rsheet for VG = 1 V (electron accumulation) decreased down to 3 kΩ upon decreasing temperature to 40 K, indicating metallic conduction. Accumulation of electrons (holes) was confirmed for VG > 1 V (VG < −1 V) by Hall effect measurements. Hall mobilities of electrons and holes, which increase with decreasing temperature, were 6500 cm2 V−1 s−1 and 570 cm2 V−1 s−1, respectively, at 120 K.

Complexation of novel multiply N-confused expanded calix[n]phyrins with boron difluoride afforded a new class of cyclic BODIPY (boron-dipyrromethene) arrays. The structures of circularly arranged BODIPY subunits linked in an N-confused fashion give rise to such photophysical properties unique to the macrocycles as redshifted emission wavelengths along with apparent large Stokes shifts, long emission lifetimes, and solid-state lasing. The DFT calculations support the size-dependent excited-state dynamics of the macrocycles.

Novel near-infrared phosphorescent iridium(III) complexes based on benzo-annulated N-linked corrole analogue (termed as benzonorrole) were synthesized. The structures of the complexes revealed octahedral coordination geometries involving an organometallic iridium–carbon bond with two external axial ligands. Interestingly, the iridium(III) complex exhibits near-infrared phosphorescence at room temperature at wavelengths beyond 900 nm. The significant redshift of the emission, as compared to the corrole congener, is originated from the ligand-centered triplet character. The fine-tuning of the photophysical properties of the complexes was achieved by introducing electron-donating and electron-withdrawing substituents on the axial pyridine ligands.

An attempt is made to mimic a system under the influence of nonradiative recombination (NR) by use of a Si 1 − x Ge x quantum well (QW) embedded in a Si 1 − x Ge x /Si strained-layer superlattice. Carrier transfer mediates the coupling between the first miniband of superlattice barrier and QW ground state, which is useful in setting up an artificial NR center based only on radiative channels. Steady-state and transient photoluminescence (PL) reproduces spectroscopic features shared by NR systems at large such as quadratic dependence of PL intensity on excitation power and prolonged decay time at increased pumping, which are quite opposite to radiative recombination.

We identified the orientation of individual Lu@C82 molecules on alkanethiol self-assembled monolayers (SAMs) by scanning tunneling microscopy (STM) at a molecular resolution. STM images of Lu@C82 on alkanethiol SAMs at 65 K showed a striped structure corresponding to the molecular orbitals of the Lu@C82 molecule, suggesting that thermal rotation of Lu@C82 on alkanethiol SAMs is prevented at 65 K. By comparing these molecular-resolution STM images with Kohn−Sham molecular orbitals of Lu@C82 calculated by density functional theory (DFT), we identified the molecular orientation of Lu@C82. Spatial mapping of the differential conductance on individual Lu@C82 molecules revealed that the local conductivity within a molecule became large around the Lu atom at a negative sample bias voltage. From spatial mapping of the differential conductance measurements, we also evaluated the HOMO−LUMO gap of Lu@C82 to be 0.47 eV. From the results of the spatial mapping of the differential conductance and DFT calculations, the ...

The interface trap level in top-contact pentacene thin-film transistors (TFTs) is obtained by displacement current measurement (DCM) during device operation at temperatures ranging from 260 to 340 K. The carrier injection voltage at the source electrode (Vinj) was measured from the displacement current and found to be independent of temperature, suggesting an Ohmic contact at the Au source electrode/pentacene interface. On the contrary, the threshold voltage (Vth) depended on the temperature. The interface trap level at the pentacene/gate dielectric interface was estimated as 36 meV from the temperature dependence of the interface trap density. By using the DCM method, it becomes possible to demonstrate the carrier injection and transport properties of top-contact pentacene TFTs.

Drain-to-source current (IDS ¼ð IDISÞ=2) and displacement current (Idis ¼ IS þ ID) are evaluated using the simultaneous measurements of source (IS) and drain (ID) currents during the application of a constant drain voltage and a triangular-wave gate voltage (VGS) to top-contact pentacene thin-film transistors. The carrier mobility, threshold voltages and mean potential drops at the source/channel and channel/drain interface are simultaneously obtained from IDS-VGS and Idis-VGS relationships. Carrier injection properties, namely, the carrier injection voltage at the source electrode and the mean potential drop-VGS relathionship are discussed on the basis of results of the simultaneous measurements of IDS and Idis. (DOI: 10.1143/JJAP.46.390)

The interaction control between endohedral metallofullerenes and a metal substrate has been demonstrated by introducing hexanethiol, octanethiol, and decanethiol self-assembled monolayers (SAMs) as the interlayer. We observe the electric properties of terbium endohedral metallofullerenes (Tb@C82) on alkanethiol SAMs with different chain lengths by scanning tunneling microscopy (STM) and spectroscopy (STS). Based on the comparison of the high-resolution STM images of a Tb@C82 molecule on hexanethiol and octanethiol SAMs, the interaction between Tb@C82 and a hexanethiol SAM is found to be larger than that between Tb@C82 and an octanethiol SAM; this is because at 68 K, the rotational states of Tb@C82 terminate only on the hexanethiol SAM. Furthermore, we find that the tunneling current-voltage characteristics of Tb@C82 on the hexanethiol SAM show the rectifying effects that are also caused by the molecular energy level shifts of Tb@C82 molecules due to the large interaction.

The single molecular orientation switching of the Tb@C82 endohedral metallofullerene has been studied by using low-temperature ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). An octanethiol self-assembled monolayer (SAM) was introduced between Tb@C82 and the Au(111) substrate to control the thermal rotational states of Tb@C82. Scanning tunneling spectroscopy (STS) of Tb@C82 on an octanethiol SAM at 13 K demonstrated hysteresis including negative differential conductance (NDC). This observed hysteresis and NDC is interpreted in terms of a switching of the Tb@C82 molecular orientation caused by the interaction between its electric dipole moment and an external electric field.

We demonstrate the gate voltage (VGS) frequency dependences of displacement current (Idis) and channel current (IDS), which were simultaneously measured under the application of a triangular wave of VGS and a constant drain voltage in top-contact pentacene organic thin-film transistors (OTFTs). Carrier injection and carrier transport properties, such as field-effect mobility, threshold voltage, and carrier injection voltages at the source and drain electrodes, were evaluated; they showed no frequency dependences at frequencies between 1 and 50 kHz. From the source current -VGS and drain current -VGS characteristics, a charge sheet at the interface of the pentacene thin-film and gate dielectric was found to form below the source and drain electrodes in this frequency range. For the practical use of OTFTs, a simultaneous measurement system for Idis and IDS is a simple and powerful technique for evaluating both carrier injection and carrier transport properties under OTFT operations.

Threshold voltage instabilities in SiO2/polyimide dual-gate dielectric pentacene thin-film transistors are investigated as a function of bias stress time for 1000 s at temperatures between 260 and 340 K in nitrogen atmosphere. Field-effect mobility maintains constant values at every measurement temperature during the application of constant bias stress voltage. The threshold voltage shift at all measurement temperatures is described by the stretched exponential stress time dependence of ΔVth(t) = ΔVth0{1-exp [-(t/τ)β]}. These experimental results suggest that our threshold voltage shift can be interpreted as carrier injection from the pentacene channel into traps located at the channel/gate dielectric interface.

The stochastic single-molecule conductance switchings of nitro-substituted oligo(phenylene–ethynylene) (OPE) molecules in three kinds of host matrices of octanethiol (C8), decanethiol (C10), and dodecanethiol (C12) self-assembled monolayers (SAMs) have been studied by scanning tunneling microscopy (STM). The stochastic single-molecule conductance switching of OPE molecules has been observed repeatedly in both the low-density striped phase of the C8 SAM and the high-density standing-up phases of the C10 and C12 SAMs by successive STM. The mean apparent height differences between the inserted OPE and the host matrices of the C8, C10, and C12 SAMs are approximately 11.9, 4.8, and 4.2 A, respectively.

Nontrivial multipass interference fringes are found in the photoluminescence spectra taken from a single-quantum-well structure in the backscattering geometry. Fringe spacing indicates light interference within the 3.3-µm-thick capping layer, whereas the transmission spectra apparently lack the corresponding fringes. This indicates that an otherwise unlikely detour of photons towards the rough backside of the 350-µm substrate is relevant, which gives rise to a 3.3-µm-thick ghost cavity. The visibility of such ghost-cavity fringes is the largest for a single emission layer and is even enhanced by forward-scattering, which can facilitate light-emitter design rather than compromising the peak assignment of spectra.

A new class of silicon-based light-emitting diode is demonstrated using InSb-quantum-dot-embedded Si containing the emissive {311} rod-like defects (RLDs). A narrow peak centered at 1377 nm (900 meV) characteristic of the {311} RLDs was found to develop out of an otherwise broad background electroluminescence (EL) upon the application of electric fields in the growth direction. Such electric-field-active EL was observed up to 150 K with a slight downward shift of the peak energies, accompanied by an anomaly in the thermal roll-off of the EL intensity. Spectral variations with temperature and electric field indicate a switching of dominance between the closely correlated defect states that are responsible for the EL emission.

We fabricated light-emitting diodes (LEDs) having an n+-Ge/p+-Si structure with ferromagnetic metal/insulator tunnel contact. Clear electroluminescence (EL) with peak wavelength around 1600 nm from the direct band gap of the Ge layer was successfully observed at room temperature, which indicates that tunnel contacts allow efficient direct-gap EL from the LED. Our result is an important step toward developing Ge-based spin-polarized LEDs.

Coulomb staircases in double-barrier tunneling junctions consisting of a scanning-probe--vacuum-gap--alkanethiol-protected Au nanoparticle/Au (111) electrode have been measured as a function of the set point current of scanning tunneling spectroscopy. The tunneling resistances of the scanning probe-Au core of a nanoparticle $({R}_{1})$ and the Au core-Au (111) electrode $({R}_{2})$ are evaluated by fitting a theoretical Coulomb staircase into the experimental tunneling current-voltage characteristics measured by scanning tunneling spectroscopy. When a vacuum gap exists between the scanning probe and alkanethiol Au nanoparticles, ${R}_{1}$ is inversely proportional to the set point current. On the contrary, in the case of ${R}_{1}l{R}_{2}$, the top of the tip of the scanning probe tends to penetrate the octanethiol-protecting molecule of an Au nanoparticle. ${R}_{2}$ is found to be independent of the set point current, and ${R}_{2}$ of octanethiol- and hexanethiol-protected Au nanoparticles are evaluated as $7.6\phantom{\rule{0.3em}{0ex}}\mathrm{G}\ensuremath{\Omega}\ifmmode\pm\else\textpm\fi{}10%$ and $460\phantom{\rule{0.3em}{0ex}}\mathrm{M}\ensuremath{\Omega}\ifmmode\pm\else\textpm\fi{}10%$, respectively.

We report measurements of the tunneling current – distance (I-d) dependence above alkanethiol self-assembled monolayers (SAMs) on Au (111) substrates with high probe voltage. From the semilogarithmic I-d plots of hexanethiol and octanethiol SAMs, a kink in the tunneling current slopes is clearly observed, which shows the point where the STM tip contacts the end of the SAMs. The conductance decay constants of the vacuum layer and the alkanethiol SAMs are estimated from the tunneling current slopes. We also discuss the contact conductance of alkanethiol SAMs.

We explored optical excitation pathways in the multivalley semiconductor Ge in an attempt to expedite selective electron injection into the indirect L-band-edge. An off-peak resonant excitation route was developed, which offers the pumping efficiency outperforming the phonon-assisted near-indirect-edge absorption by more than six orders of magnitude. The valley selectivity results from the intra-valley relaxation that separates electrons and holes in momentum space following excitation. Fortuitously, the widely used green laser, 532 nm, is found to be nearly ideally suited to the efficient L-valley-selective excitation in Ge. Such valley-specific pumping may help clarify the otherwise complicated electron dynamics involving intervalley processes.

Direct band gap shrinkage in Ge-on-insulator (GOI) structures fabricated by lateral liquid-phase epitaxy (LLPE) was investigated by means of micro(μ)-photoluminescence and μ-Raman spectroscopy. The LLPE method, based on the rapid thermal process, was found to be an effective and feasible way to produce highly strained local GOI structures. We observed a significant redshift of direct gap emission amounting to 45 meV from the tensile-strained GOI layer. Strain analysis and temperature dependent PL spectra indicated that a direct band gap shrinkage was mainly due to a tensile strain of about 0.4% induced by rapid crystallization from the Ge melting point during the LLPE process. The local optical properties along the LLPE-grown GOI wire were examined and discussed on the basis of μ-PL mapping images.

Non-zero spin polarization induced by optical orientation was clearly observed at room temperature in a Ge/Ge0.8Si0.2 quantum well grown on Ge and a Ge layer grown on Si by molecular beam epitaxy, whereas it was absent in bulk Ge. This occurred because indirect-gap photoluminescence (PL), which can obscure the spin-polarization information carried by the direct-gap PL, was quenched by unintentional growth-related defects in the epitaxial layers. Such interpretation was confirmed by applying time gating that effectively removed the indirect-gap PL characterized by a slower rise time, which allowed us to demonstrate the existence of room-temperature spin polarization in bulk Ge.

Time-resolved spectroscopy of bulk Ge at room temperature allows to differentiate direct-gap electroluminescence (EL) from indirect-gap EL. The results indicate that there exists strong coupling between the eight-fold indirect L-valleys and the zone-center Γ-valley due to intervalley phonon scattering at room temperature in the absence of built-in strain. Pulse modulation of EL at 10-MHz is consistent with the relevance of the direct-gap transition.

We studied the Bi wire-δ-doping process to achieve a high concentration of Bi donors in Si. Our process has two steps: (i) burial of Bi nanowires in Si by molecular beam epitaxy, and (ii) activation of Bi atoms in the δ-doped layer by laser annealing. The peak concentration of Bi atoms in the δ-doped layer is controlled by two parameters: the coverage of surfactant layer, and the growth temperature during the Si cap-layer growth, whose maximum concentration is larger than 1020 cm-3. Photoluminescence and electrical carrier transport measurements reveal that dense Bi atoms are activated upon heating the area at close to the melting point of Si. As a result, our doping process results in Bi donors in the wire-δ-doped layer with concentration of >1018 cm-3. This will be useful for establishing next-generation, quantum information processing platform.

We propose a new method of creating light-emitting point defects, or G-centers, by modifying a silicon surface with hexamethyldisilazane followed by laser annealing of the surface region. This laser annealing process has two advantages: creation of highly dense G-centers by incorporating carbon atoms into the silicon during heating; freezing in the created G-centers during rapid cooling. The method provides a surface region of up to 200 nm with highly dense carbon atoms of up to 4 × 1019 cm−3 to create G-centers, above the solubility limit of carbon atoms in silicon crystal (3 × 1017 cm−3). Photoluminescence measurement reveals that the higher-speed laser annealing produces stronger G-center luminescence. We demonstrate electrically-driven emission from the G-centers in samples made using our new method.

Coulomb diamonds were clearly observed on single-electron transistors (SETs) fabricated by bottom-up processes of electroless plating of Au nanogap electrodes and chemisorption of a Au nanoparticle at 80 K. In the drain current–drain voltage characteristics, Coulomb staircases were modulated by the side gate voltage. Tunneling resistances and source/drain/gate capacitances of the SET were evaluated by fitting the theoretical Coulomb staircase determined on the basis of the full orthodox theory in a double-barrier tunneling junction to the experimental results of Coulomb blockade under the application of side gate voltages. The theoretical results for the Coulomb diamond are in good agreement with the experimental results.

Alkanethiol-protected Au nanoparticles have been chemically immobilized on Au(111) substrates by alkanedithiol and alkanethiol mixed self-assembled monolayers (SAMs). Octanethiol SAMs on Au(111) have been used as host matrices for molecular insertion by 1,8-octanedithiol (C8S2) and 1,10-decanedithiol (C10S2). The dithiol molecules in the mixed SAMs and the Au nanoparticles on the mixed SAMs were observed by scanning probe microscopy. The densities of C8S2 and C10S2 mixed SAMs were controlled by varying the dithiol concentration from 0.5 to 50 mM. The densities of Au nanoparticles were almost equal to those of the dithiol molecules, indicating that the Au nanoparticle chemisorbed through the protruding sulfur of the dithiol molecules displaced the protecting group of the Au core. The chemisorbed Au nanoparticles were stable for scanning tunneling spectroscopy (STS) measurements. The resistance of the chemisorbed nanoparticles between the Au core and the Au(111) substrate is discussed.

We report a simple and high yield method for fabricating multiple nanogaps simultaneously by an electroless gold plating technique using electroless gold plating solution which consists of common medical liquid of iodine tincture and L(+)-ascorbic acid (vitamin C). The distance between the gold electrodes (33nm in average) on the SiO2∕Si substrate was decreased by selective deposition of gold onto the surface of the gold electrodes. By electroless gold plating, we fabricated nanogaps below 5nm in width with a 41% process yield. We also demonstrated the Coulomb blockade effect in octanethiol(C8)-protected Au nanoparticles by using such a fabricated nanogap.

We demonstrate a Si-based light-emitting diode (LED) with MHz modulation capability containing a certain class of light-emitting defects in the active region. The emission spectra are characterized by a narrow peak centered at 900 meV (1377 nm), hereafter referred to as E-line, which is attributed to the {311} defects inherent to cubic crystals with the diamond sublattice. The luminescence intensity of the E-line was found to go through a maximum around 20K persisting above 150 K. In spite of the decay lifetimes even longer than 1μs, 31% amplitude modulation was achieved for 1-MHz rectangular pulses at 20 K while near 10-MHz bandwidth was obtained for dynamic operation with modulated pulse widths.