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In search of heavy metal-free mid-IR active colloidal materials, self-doped silver selenide colloidal quantum dots (CQDs) can be an alternative offering tunable mid-IR wavelength with a narrow bandwidth. One of the challenges in the study of the intraband transition is developing a method to widen the intraband transition energy range as well as reducing the toxicity of the materials. Here, we present Ag

Intraband transition and localized surface plasmon resonance of metal chalcogenide nanocrystals that are sensitive to the crystal structure.

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

In this opinion we trace the evolution of the quantum dot mid-infrared photodetector, from epitaxially-grown self-assembled quantum dot detectors, to a new generation of colloidal nano-crystal based devices. We opine on the advantages and challenges associated with these colloidal quantum dot materials and discuss their potential for commercial device applications.

Solution-processable perovskite quantum dots are considered as promising optical materials for light-emitting optoelectronics. Light-emitting field-effect transistors (LEFETs) that can be operated under a relatively lower potential with a high energy conversion efficiency are yet to be realized with perovskite quantum dots. Here, we present the CsPbBr

Realizing bright colloidal infrared emitters in the midwavelength infrared (or mid-IR), which can be used for low-power IR light-emitting diodes (LEDs), sensors, and deep-tissue imaging, has been a challenge for the last few decades. Here, we present colloidal tellurium nanowires with strong emission intensity at room temperature and even lasing at 3.6 μm (ω) under cryotemperature. Furthermore, the second-harmonic field at 1.8 μm (2ω) and the third-harmonic field at 1.2 μm (3ω) are successfully generated thanks to the intrinsic property of the tellurium nanowire. These unique optical features have never been reported for colloidal tellurium nanocrystals. With the colloidal midwavelength infrared (MWIR) Te nanowire laser, we demonstrate its potential in biomedical applications. MWIR lasing has been clearly observed from nanowires embedded in a human neuroblastoma cell, which could further realize deep-tissue imaging and thermotherapy in the near future.

Demands on nontoxic nanomaterials in the short-wavelength infrared (SWIR) have rapidly grown over the past decade. Here, we present the nonstoichiometric silver chalcogenide nanocrystals of AgxTe (x > 2) and Ag2Te/Ag2S CQDs with a tunable bandgap across the SWIR region. When the atomic percent of the metal and chalcogenide elements are varied, the emission frequency of the excitonic peak is successfully extended to 2.7 μm. Surprisingly, the AgxTe CQD film responds to the SWIR light with a responsivity of 2.1 A/W at 78 K. Also, the Ag2S shell growth over the Ag2Te core enhances not only the emission intensity but also the structural rigidity, preventing crystal morphology deformation under the electron beam. The origin of the enhancement in the emission intensity and air stability of AgxTe and Ag2Te/Ag2S CQDs is carefully investigated by X-ray photoelectron spectroscopy (XPS). The optical properties and infrared photocurrent of AgxTe CQDs will provide new opportunities for solution-based SWIR applications.

Colloidal quantum dots, which have demonstrated its usage in optoelectronics in the visible range, are promising as infrared active materials as well. The wavelength-tunability by many variables including the nanocrystal size allows easy access to the target wavelength. Mostly, the bandgap transition is used for energy conversion. Self-doped quantum dots are the nanocrystal in which other electronic transitions are available under steady-state and ambient conditions. This talk will focus on the synthesis of the self-doped Ag2Se, HgSe QDs and their various properties including carrier dynamics studied by mid-IR spectroscopy. Additionally, several applications based on the intraband transition will be discussed.

Intraband transitions of colloidal semiconductor nanocrystals, or the electronic transitions occurring in either the conduction band or valence band, have recently received considerable attention because utilizing the intraband transitions provides new approaches for applications such as photodetectors, imaging, solar cells, lasers, and so on. In the past few years, it has been revealed that observing the intraband transition is not limited for temporal measurement such as ultrafast spectroscopy but available for steady-state measurement even under ambient conditions with the help of self-doped semiconductor nanocrystals. Considering the large absorption coefficient of the steady-state intraband transition comparable to that of the bandgap transition, the use of the intraband transition will be promising for both fundamental and application studies. Here, we summarize the recent progress in studies on intraband photoluminescence of self-doped semiconductor nanocrystals and discuss key questions to be addressed in future research.

We report that high absorption PbSe colloidal quantum dots (QDs) having a peak absorbance beyond 2100 nm were synthesized and incorporated into InSnZnO (ITZO) channel layer-based thin film transistors (TFTs). It was intended that PbSe QDs with proportionally less photocurrent modulation can be remedied by semiconducting and low off-current ITZO-based TFT configuration. Multiple deposition scheme of PbSe QDs on ITZO metal oxide thin film gave rise to nearly linear increase of film thickness with acceptably uniform and smooth surface (less than 10 nm). Hybrid PbSe/ITZO thin film-based phototransistor exhibited the best performance of near infrared (NIR) detection in terms of response time, sensitivity and detectivity as high as 0.38 s, 3.91 and 4.55 × 107 Jones at room temperature, respectively. This is indebted mainly from the effective diffusion of photogenerated carrier from the PbSe surface to ITZO channel layer as well as from the conduction band alignment between them. Therefore, we believe that our hybrid PbSe/ITZO material platform can be widely used to be in favour of incorporation of solution-processed colloidal light absorbing material into the high-performance metal oxide thin film transistor configuration.

Steady-state intraband transition, which is a promising electronic transition of a colloidal quantum dot along with the band-gap transition, had been a long-standing challenge. The steady-state intraband transition occurring between discrete electronic states in the conduction band of a colloidal nanocrystal has been reported only from mercury chalcogenide nanocrystals for the past few years. Concerns about the toxicity of the mercury compound necessitate a new nontoxic system exhibiting a steady-state intraband transition. Here we present the steady-state intraband absorption and photoluminescence of Ag2Se colloidal nanocrystals under ambient conditions. The mid-IR intraband transition is carefully investigated by means of FT-IR emission spectroscopy, spectroelectrochemistry, compositional analysis, and transfer characteristics. Especially, the mid-IR intraband photoluminescence of the Ag2Se colloidal nanocrystal will open new avenues in the use of quantum-confined colloidal systems for mid- and long-wav...

The tunable bandgap energy has been recognized as a prominent feature of the colloidal semiconductor nanocrystal, also called the colloidal quantum dot (CQD). Due to the broken degeneracy caused by the quantum confinement effect, the electronic states of the conduction band (CB) are separated by a few hundred meV. The electronic transition occurring in the conduction band is called the intraband transition and has been regarded as a fast electron relaxation process that cannot be readily observed under steady state. However, recent progress in the studies of intraband transitions allowed the observation of the mid-IR intraband transition in steady state and ambient condition, providing a pathway to exploit the mid-IR electronic transition for various optoelectronic applications. The observation of the steady state intraband transitions has been possible due to the electron filling of the lowest electronic state (1Se) of the conduction band in the semiconductor nanocrystal. Specifically, the nanocrystals are "self-doped" with electrons through chemical synthesis - that is, without the need of adding heterogeneous impurity or applying an electrical potential. In this feature article, we summarize the recent advances in the study on intraband electronic transitions along with the interesting findings on the magnetic and electronic properties of the self-doped colloidal metal chalcogenide semiconductor nanocrystals. The mid-IR intraband transitions of non-toxic nanocrystals, which exclude the toxic mercury and cadmium constituents, are also highlighted, which hold promise for safer applications utilizing the higher quantum states of nanocrystals.

CdxHg1–xSe alloy nanocrystals are obtained from CdSe semiconductor nanocrystals via cation exchange. By varying the composition during the exchange process, the CdxHg1–xSe alloy nanocrystals offer a widely tunable electronic transition from visible to NIR and even to mid-IR range. The visible bandgap transition of the CdSe colloidal quantum dot gradually red-shifts to the near-IR with the addition of the Hg precursor, and then the steady-state intraband (or intersub-band) transition of the CdxHg1–xSe alloy nanocrystals appears. Finally, as the electron density is increased by successive addition of metal precursor, localized surface plasmon resonances (LSPRs) appear as a major electronic transition in the mid-IR regime. The shift of the major electronic transition from the bandgap to LSPRs infers that the exciton spatially moves to the surface from the inside of the nanocrystal through the cation change and further crystal growth. The corresponding variance of the nanocrystals’ structural, compositional, ...

Filling the lowest quantum state of the conduction band of colloidal nanocrystals with a single electron, which is analogous to the filling the lowest unoccupied molecular orbital in a molecule with a single electron, has attracted much attention due to the possibility of harnessing the electron spin for potential spin-based applications. The quantized energy levels of the artificial atom, in principle, make it possible for a nanocrystal to be filled with an electron if the Fermi-energy level is optimally tuned during the nanocrystal growth. Here, we report the singly occupied quantum state (SOQS) and doubly occupied quantum state (DOQS) of a colloidal nanocrystal in steady state under ambient conditions. The number of electrons occupying the lowest quantum state can be controlled to be zero, one (unpaired), and two (paired) depending on the nanocrystal growth time via changing the stoichiometry of the nanocrystal. Electron paramagnetic resonance spectroscopy proved the nanocrystals with single electron t...

Tellurium has been of great interest in physics, chemistry, material science, and more recently in nanoscience. However, information on the photoluminescence of Te crystals, crucial in understanding the material, has never been disclosed. Here, we present photoluminescence and lasing for the Te bulk crystal and microcrystals. Photoluminescence of Te bulk solid crystal was observed at 3.75 μm in the midwavelength infrared (MWIR) region, matching the theoretically predicted value well. With increasing the photoexcitation intensity or decreasing temperature, we successfully observed MWIR random lasing of the bulk Te crystals at 3.62 μm. Furthermore, the rod-shaped Te microcrystals efficiently exhibit second harmonic and third harmonic lasing at MWIR and short-wavelength infrared regions, respectively. Nonlinear coherent MWIR lasing from the Te microcrystals will serve as an excellent mid-IR light source, opening up new applications in infrared photonics, extremely long-depth penetration bioimaging, and optoelectronics.

Self-doping in nanocrystals allows accessing higher quantum states. The electrons occupying the lowest energy state of the conduction band form a metastable state that is very sensitive to the electrostatic potential of the surface. Here, we demonstrate that the high charge sensitivity of the self-doped HgSe colloidal quantum dot solid can be used for sensing three different targets with different phases through self-doped HgSe nanocrystal/ZnO thin-film transistors: the environmental gases (CO2 gas, NO gas, and H2S gas); mid-IR photon; and biothiol (l-cysteine) molecules. The self-doped quantum dot solid detects the targets through different mechanisms. The physisorption of the CO2 gas and the NO gas molecules, and the mid-IR photodetection show reversible processes, whereas the chemisorption of l-cysteine biothiol and H2S gas molecules shows irreversible processes. Considering the quenching of mid-IR intraband photoluminescence of the HgSe colloidal quantum dot solid by the vibrational mode of the CO2 ga...

Colloidal quantum dots have been used in various research fields such as display, bio-imaging, therapy, detectors, etc. Mostly, the bandgap electronic transitions are used to convert the light to electrical signals or vice versa. Not only the bandgap transition, but also the other electronic transitions are available under certain conditions , which is a merit of the quantum materials. The intraband transition, recently demonstrated in steady state, also can be used for various applications. To utilize the intraband transition, the quantum dots should be self-doped and the doped state should last even under ambient conditions. This talk will focus on optical and electrical properties of self-doped silver chalcogenide colloidal quantum dots exhibiting mid-IR intraband electronic transitions in steady-state.

Electron occupation in the lowest quantized state of the conduction band (1Se) in the colloidal quantum dot leads to the intraband transition in steady-state (1Se-1Pe). The intraband transition, solely originating from the quantum confinement effect, is the unique property of semiconducting nanocrystals. To achieve the electron occupation in 1Se state in the absence of impurity ions, nonthiol ligand passivated HgS colloidal quantum dots are synthesized. The nonthiol ligand passivated HgS quantum dot exhibits strong steady-state intraband transition in ambient condition and enables a versatile ligand replacement to oxide, acid, and halide functional ligands, which was not achievable from conventional HgS or HgSe quantum dots. Surprisingly, the atomic ligand passivation to HgS colloidal quantum dot solution efficiently maintains the electron occupation at 1Se of HgS CQDs in ambient condition. The electron occupation in 1Se of HgS CQD solid film is controlled by surface treatment with charged ions, which is ...

Mid-infrared intraband photoluminescence is observed from CdSe and CdS colloidal quantum dots (CQDs) and core/shell systems when excited by a visible laser. The CQDs show more intraband photoluminescence with dodecanethiol than with other ligands. Core/shells show an increase of the intraband photoluminescence with increasing shell thickness. The detected emission is restricted to below 2900 cm(-1), bounded by the C-H vibrational modes of the organic ligands. Upon photoexcitation in air for all dodecanethiol ligands capped CQD systems studied, the intraband photoluminescence is quenched over time, and emission at lower frequency is observed, which is assigned to laser heating and thermal emission from oxides.

Amyloid fibrils have recently been highlighted for their diverse applications as functional nanomaterials in modern chemistry. However, tight control to obtain a targeted fibril length with low heterogeneity has not been achieved because of the complicated nature of amyloid fibrillation. Herein, we demonstrate that fibril assemblies can be homogeneously manipulated with desired lengths from ~40 nm to ~10 μm by a phase transfer of amyloid proteins based on host-guest chemistry. We suggest that host-guest interactions with cucurbit[6]uril induce a phase transfer of amyloid proteins (human insulin, human islet amyloid polypeptide, hen egg lysozyme, and amyloid-β 1–40 & 1–42) from the soluble state to insoluble state when the amount of cucurbit[6]uril exceeds its solubility limit in solution. The phase transfer of the proteins kinetically delays the nucleation of amyloid proteins, while the nuclei formed in the early stage are homogeneously assembled to fibrils. Consequently, supramolecular assemblies of amyloid proteins with heterogeneous kinetics can be controlled by protein phase transfer based on host-guest interactions.

Photoconductivity is demonstrated with monodispersed HgSe colloidal quantum dots that are illuminated with radiation resonant with 1S(e)-1P(e) intraband electronic absorption, between 3 and 5 μm. A doping of two electrons per dot gives the lowest dark current, and a detectivity of 8.5 × 10(8) Jones is obtained at 80 K. Photoluminescence of the intraband transition is also observed. The detector properties are discussed in terms of the measured photoluminescence quantum yield, the electron mobility in the 1P(e) state, and the responsivity. The intraband photoresponse allows to fully harness the quantum confined states in colloidal nanostructures, extending the prior limited use of interband transition.

Molecules affect the electronic properties of many emerging materials, ranging from organic thin film transistors and light emitting diodes for flexible displays to colloidal quantum dots (CQDs) used in solution processed photovoltaics and photodetectors. For example, the interactions of conjugated molecules not only influence morphological and charge transport properties of organic photovoltaic (OPV) materials, but they also determine the primary photophysical events leading to charge generation. Ligand-nanocrystal interactions affect the density and energetic distributions of trap states, which in turn influence minority carrier transport in CQD photovoltaics. Therefore, it is critical for scientists to understand how the underlying molecular structures and morphologies determine the electronic properties of emerging materials. Recently, chemists have used vibrational spectroscopy to study electronic processes in emerging materials, and been able to directly measure the influence molecular properties have on those processes. Time-resolved vibrational spectroscopy is uniquely positioned to examine molecular species involved in electronic processes because it combines ultrafast time resolution with measurement of the vibrational spectra of molecules. For instance, molecules at the electron donor/acceptor interfaces in OPV materials have unique vibrational features because vibrational frequencies of molecules are sensitive to their local molecular environments. Through ultrafast vibrational spectroscopy, researchers can directly examine the dynamics of charge transfer (CT) state formation and dissociation to form charge separated states specifically at donor/acceptor interfaces. Vibrational modes of ligands are also sensitive to their bonding interactions with nanocrystal surfaces, which enables chemists to directly probe the molecular nature of charge trap states in colloidal quantum dot solids. Because of the ability to connect electrical properties with the underlying molecular species, scientists can use ultrafast vibrational spectroscopy to address fundamental challenges in the development of emerging electronic materials. In this Account, we focus on two applications of vibrational spectroscopy to examine electronic processes in OPV and CQD photovoltaic materials. In the first application, we examine archetypal classes of electron acceptors in OPV materials and reveal how their molecular structures influence the dynamics and energetic barriers to CT state formation and dissociation. In the second application, we discuss the surface chemistry of ligand-nanocrystal interactions and how they impact the density and energetic distribution of charge trap states in CQD photovoltaic materials. Through direct observations of the vibrational features of ligands attached to surface trap states, we can obtain valuable insights into the nature of charge traps and begin to understand pathways for their elimination. We expect that further examination of electronic processes in materials using ultrafast vibrational spectroscopy will lead to new design rules in support of continued materials development efforts.

Fullerenes, cage-structured carbon allotropes, have been the subject of extensive research as new materials for diverse purposes. Yet, their formation process is still not clearly understood at the molecular level. In this study, we performed laser desorption ionization-ion mobility-mass spectrometry (LDI-IM-MS) of carbon substrates possessing different molecular sizes and structures to understand the formation process of fullerene. Our observations show that the formation process is strongly dependent on the size of the precursor used, with small precursors yielding small fullerenes and large graphitic precursors generally yielding larger fullerenes. These results clearly demonstrate that fullerene formation can proceed via both bottom-up and top-down processes, with the latter being favored for large precursors and more efficient at forming fullerenes. Furthermore, we observed that specific structures of carbon precursors could additionally affect the relative abundance of C60 fullerene. Overall, this study provides an advanced understanding of the mechanistic details underlying the formation processes of fullerene.

Colloidal quantum dot (CQD) photovoltaics offer a promising approach to harvest the near-IR region of the solar spectrum, where half of the sun's power reaching the earth resides. High external quantum efficiencies have been obtained in the visible region in lead chalcogenide CQD photovoltaics. However, the corresponding efficiencies for band gap radiation in the near-infrared lag behind because the thickness of CQD photovoltaic layers from which charge carriers can be extracted is limited by short carrier diffusion lengths. Here, we investigate, using a combination of electrical and optical characterization techniques, ligand passivation strategies aimed at tuning the density and energetic distribution of charge trap states at PbS nanocrystal surfaces. Electrical and optical measurements reveal a more than 7-fold enhancement of the mobility-lifetime product of PbS CQD films treated with 3-mercaptopropionic acid (MPA) in comparison to traditional organic passivation strategies that have been examined in the literature. We show by direct head-to-head comparison that the greater mobility-lifetime products of MPA-treated devices enable markedly greater short-circuit current and higher power conversion efficiency under AM1.5 illumination. Our findings highlight the importance of selecting ligand treatment strategies capable of passivating a diversity of surface states to enable shallower and lower density trap distributions for better transport and more efficient CQD solar cells.

HgS nanocrystals show a strong mid-infrared absorption and a bleach of the near-infrared band edge, both tunable in energy and reversibly controlled by exposure to solution ions under ambient conditions. The same effects are obtained by applying a reducing electrochemical potential, confirming that the mid-infrared absorption is the intraband transition of the quantum dot. This is the first time that stable carriers are present in the quantum state of strongly confined quantum dot in ambient conditions. The mechanism by which doping is achieved is attributed to the rigid shifts of the valence and conduction band with respect to the environment, similar to the sensitivity of the work function of surfaces to adsorbates.

The electrical and frictional properties of axially grooved oil-impregnated sintered-metal bearing (porous metal bearing) with varying loads and speeds were measured. The results were compared with the performance of bearings without grooves and conventionally used ball bearings. The test results showed that the frictional values of the porous bearings were lower than those of ungrooved bearings and ball bearings, and that the porous bearings operated in a full-hydrodynamic lubrication regime. The values of rating current, starting time and jitter revealed that the grooved-bearing had better electrical characteristics than the ungrooved bearing as well as the ball bearing. From the endurance tests on start-stop and continuous operating, it was confirmed that this bearing could be mass-produced for use in a laser scanner unit. Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999

The frictional characteristics of oil-impregnated sintered-metal bearing (OSB) and herringbone-grooved fluid bearing (HFB) with varying loads and speeds were measured, and their electrical properties assembled in the scanner motor were also measured to develop new journal bearings, which could overcome the speed limit of ball bearing. OSB shows very good performance up to 20,000 rpm, and HFB successfully works up to 35,000 rpm. From the endurance tests on start-stop and continuous operating, it was confirmed that those bearings could satisfy the demand for high-rotating technology and could be mass-produced for use on high-speed scanner motor in a laser scanner unit. [S0742-4787(00)02703-X]

Enhancing the dielectric permittivity of organic semiconductors may open new opportunities to control charge generation and recombination dynamics in organic solar cells. The potential to tune the dielectric permittivity of organic semiconductors by doping them with redox inactive salts was explored using a combination of organic synthesis, electrical characterization, and time-resolved infrared spectroscopy. The addition of the salt, LiTFSI (lithium bis(trifluoro-methyl-sulfonyl)imide), to a conjugated polymer specifically designed to incorporate ions into its bulk phase increased the density of holes and enhanced the static dielectric permittivity of the polymer blend by more than an order of magnitude. The frequency and phase dependence of the real dielectric function demonstrates that the increase in dielectric permittivity resulted from dipolar motion of bound ion pairs or clusters of ions. Interestingly, the increases in the hole density and dielectric permittivity were associated with enhancement of the hole mobility by 2 orders of magnitude relative to the undoped polymer. The charge recombination lifetime also increased by an order of magnitude in the blend with a fullerene electron acceptor when ions were added to the polymer. The findings indicate that ion doping enables organic semiconductors with large increases in low frequency dielectric permittivity and that these changes result in improved charge transport and suppressed charge recombination on the microsecond time scale.