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1.
J Phys Chem Lett ; : 2303-2307, 2020 Mar 09.
Artigo em Inglês | MEDLINE | ID: mdl-32102543

RESUMO

HgSe colloidal quantum dot films are made by using a hybrid ligand exchange (HgSe/hybrid) in polar inks and compared with the solid-state ligand exchange using ethanedithiol (HgSe/EDT). In both systems, the conductance shows a peak at one-electron filling of the 1Se state and a dip at 2 electrons before filling the 1Pe state. The HgSe/hybrid films show a ∼100-fold increased mobility, reaching up to ∼1 cm2/Vs for 7.5 nm diameter particles. While field effect transistor and Hall measurements give similar carrier density and mobility, the temperature dependence of the mobility is consistent with hopping transport.

2.
Adv Mater ; : e1906590, 2020 Jan 20.
Artigo em Inglês | MEDLINE | ID: mdl-31957096

RESUMO

Three-dimensional (3D) subwavelength nanostructures have emerged and triggered tremendous excitement because of their advantages over the two-dimensional (2D) counterparts in fields of plasmonics, photonic crystals, and metamaterials. However, the fabrication and integration of 3D nanophotonic structures with colloidal quantum dots (CQDs) faces several technological obstacles, as conventional lithographic and etching techniques may affect the surface chemistry of colloidal nanomaterials. Here, the direct fabrication of functional quasi-3D nanophotonic structures into CQD films is demonstrated by one-step imprinting with well-controlled precision in both vertical and lateral directions. To showcase the potential of this technique, diffraction gratings, bilayer wire-grid polarizers, and resonant metal mesh long-pass filters are imprinted on CQD films without degrading the optical and electrical properties of CQD. Furthermore, a dual-diode CQD detector into an unprecedented mid-wave infrared two-channel polarization detector is functionalized by embedding an imprinted bilayer wire-grid polarizer within the CQDs. The results show that this approach offers a feasible pathway to combine quasi-3D nanostructures with colloidal materials-based optoelectronics and access a new level of light manipulation.

3.
Nat Mater ; 2020 Jan 27.
Artigo em Inglês | MEDLINE | ID: mdl-31988516

RESUMO

Improving charge mobility in quantum dot (QD) films is important for the performance of photodetectors, solar cells and light-emitting diodes. However, these applications also require preservation of well defined QD electronic states and optical transitions. Here, we present HgTe QD films that show high mobility for charges transported through discrete QD states. A hybrid surface passivation process efficiently eliminates surface states, provides tunable air-stable n and p doping and enables hysteresis-free filling of QD states evidenced by strong conductance modulation. QD films dried at room temperature without any post-treatments exhibit mobility up to µ ~ 8 cm2 V-1 s-1 at a low carrier density of less than one electron per QD, band-like behaviour down to 77 K, and similar drift and Hall mobilities at all temperatures. This unprecedented set of electronic properties raises important questions about the delocalization and hopping mechanisms for transport in QD solids, and introduces opportunities for improving QD technologies.

4.
Nat Commun ; 10(1): 4511, 2019 10 04.
Artigo em Inglês | MEDLINE | ID: mdl-31586067

RESUMO

Colloidal quantum wells are two-dimensional materials grown with atomically-precise thickness that dictates their electronic structure. Although intersubband absorption in epitaxial quantum wells is well-known, analogous observations in non-epitaxial two-dimensional materials are sparse. Here we show that CdSe nanoplatelet quantum wells have narrow (30-200 meV), polarized intersubband absorption features when photoexcited or under applied bias, which can be tuned by thickness across the near-infrared (NIR) spectral window (900-1600 nm) inclusive of important telecommunications wavelengths. By examination of the optical absorption and polarization-resolved measurements, the NIR absorptions are assigned to electron intersubband transitions. Under photoexcitation, the intersubband features display hot carrier and Auger recombination effects similar to excitonic absorptions. Sequenced two-color photoexcitation permits the sub-picosecond modulation of the carrier temperature in such colloidal quantum wells. This work suggests that colloidal quantum wells may be promising building blocks for NIR technologies.

5.
ACS Nano ; 13(9): 10512-10519, 2019 Sep 24.
Artigo em Inglês | MEDLINE | ID: mdl-31436950

RESUMO

Transient infrared photoluminescence upconversion is used to study the exciton dynamics in small-gap HgSe colloidal quantum dots in the 2000-6500 cm-1 (0.25-0.80 eV) range. The intraband mid-infrared photoluminescence decays show absent or greatly reduced Auger relaxation of biexcitons, proposed as a generic feature of weakly n-type quantum dots due to the sparse density of states in the conduction band. The nonradiative relaxation of the intraband carriers is instead consistent with near-field energy transfer to molecular vibrations of the surface ligands. In contrast, the interband near-infrared photoluminescence decays exhibit the typical distinct exciton and biexciton lifetimes with Auger coefficients comparable to other similarly sized quantum dots. Also observed are spectral and dynamical evidence of fine structure in the intraband transitions consistent with spin-orbit splitting of the electron P levels, and the emergence of plasmonic resonances in large particles.

6.
Small ; 15(12): e1804920, 2019 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-30767425

RESUMO

Electronic eye cameras are receiving increasing interest due to their unique advantages such as wide field of view, low aberrations, and simple imaging optics compared to conventional planar focal plane arrays. However, the spectral sensing ranges of most electronic eyes are confined to the visible, which is limited by the energy gaps of the sensing materials and by fabrication obstacles. Here, a potential route leading to infrared electronic eyes is demonstrated by exploring flexible colloidal quantum dot (CQD) photovoltaic detectors. Benefitting from their tunable optical response and the ease of fabrication as solution processable materials, mercury telluride (HgTe) CQD detectors with mechanical flexibility, wide spectral sensing range, fast response, and high detectivity are demonstrated. A strategy is provided to further enhance the light absorption in flexible detectors by integrating a Fabry-Perot resonant cavity. Integrated short-wave IR detectors on flexible substrates have peak D* of 7.5 × 1010 Jones at 2.2 µm at room temperature and promise the development of infrared electronic eyes with high-resolution imaging capability. Finally, infrared images are captured with the flexible CQD detectors at varying bending conditions, showing a practical approach to sensitive infrared electronic eyes beyond the visible range.

7.
ACS Nano ; 12(9): 9397-9404, 2018 Sep 25.
Artigo em Inglês | MEDLINE | ID: mdl-30125488

RESUMO

HgTe colloidal quantum dots (QDs) are of interest because quantum confinement of semimetallic bulk HgTe allows one to synthetically control the bandgap throughout the infrared. Here, we synthesize highly monodisperse HgTe QDs and tune their doping both chemically and electrochemically. The monodispersity of the QDs was evaluated using small-angle X-ray scattering (SAXS) and suggests a diameter distribution of ∼10% across multiple batches of different sizes. Electron-doped HgTe QDs display an intraband absorbance and bleaching of the first two excitonic features. We see splitting of the intraband peaks corresponding to electronic transitions from the occupied 1Se state to a series of nondegenerate 1Pe states. Spectroelectrochemical studies reveal that the degree of splitting and relative intensity of the intraband features remain constant across doping levels up to two electrons per QD. Theoretical modeling suggests that the splitting of the 1Pe level arises from spin-orbit coupling and reduced QD symmetry. The fine structure of the intraband transitions is observed in the ensemble studies due to the size uniformity of the as-synthesized QDs and strong spin-orbit coupling inherent to HgTe.

8.
ACS Nano ; 12(7): 7264-7271, 2018 Jul 24.
Artigo em Inglês | MEDLINE | ID: mdl-29975502

RESUMO

Colloidal quantum dots (CQDs) with a band gap tunable in the mid-wave infrared (MWIR) region provide a cheap alternative to epitaxial commercial photodetectors such as HgCdTe (MCT) and InSb. Photoconductive HgTe CQD devices have demonstrated the potential of CQDs for MWIR photodetection but face limitations in speed and sensitivity. Recently, a proof-of-concept HgTe photovoltaic (PV) detector was realized, achieving background-limited infrared photodetection at cryogenic temperatures. Using a modified PV device architecture, we report up to 2 orders of magnitude improvement in the sensitivity of the HgTe CQD photodetectors. A solid-state cation exchange method was introduced during device fabrication to chemically modify the interface potential, leading to an order of magnitude improvement of external quantum efficiency at room temperature. At 230 K, the HgTe CQD photodetectors reported here achieve a sensitivity of 109 Jones with a cutoff wavelength between 4 and 5 µm, which is comparable to that of commercial photodetectors. In addition to the chemical treatment, a thin-film interference structure was devised using an optical spacer to achieve near unity internal quantum efficiency upon reducing the operating temperature. The enhanced sensitivity of the HgTe CQD photodetectors reported here should motivate interest in a cheap, solution-processed MWIR photodetector for applications extending beyond research and military defense.

9.
ACS Nano ; 12(7): 7362-7370, 2018 Jul 24.
Artigo em Inglês | MEDLINE | ID: mdl-29985583

RESUMO

Thermal imaging in the midwave infrared plays an important role for numerous applications. The key functionality is imaging devices in the atmospheric window between 3 and 5 µm, where disturbance from fog, dust, and other atmospheric influence could be avoided. Here, we demonstrate sensitive thermal imaging with HgTe colloidal quantum dot (CQD) photovoltaic detectors by integrating the HgTe CQDs with plasmonic structures. The responsivity at 5 µm is enhanced 2- to 3-fold over a wide range of operating temperatures from 295 to 85 K. A detectivity of 4 × 1011 Jones is achieved at cryogenic temperature. The noise equivalent temperature difference is 14 mK at an acquisition rate of 1 kHz for a 200 µm pixel. Thermal images are captured with a single-pixel scanning imaging system.

10.
J Phys Chem Lett ; 9(9): 2208-2211, 2018 May 03.
Artigo em Inglês | MEDLINE | ID: mdl-29648452

RESUMO

The biexciton lifetimes in HgTe colloidal quantum dots are measured as a function of particle size. Samples produced by two synthetic methods, leading to partially aggregated or well-dispersed particles, exhibit markedly different dynamics. The relaxation characteristics of partially aggregated HgTe inhibit reliable determinations of the Auger lifetime. In well-dispersed HgTe quantum dots, the biexciton lifetime increases approximately linearly with particle volume, confirming trends observed in other systems. The extracted Auger coefficient is three orders of magnitude smaller than that for bulk HgCdTe materials with similar energy gaps. We discuss these findings in the context of understanding Auger relaxation in quantum-confined systems and their relevance to mid-infrared optoelectronic devices based on HgTe colloidal quantum dots.

11.
J Phys Chem Lett ; 8(10): 2224-2228, 2017 May 18.
Artigo em Inglês | MEDLINE | ID: mdl-28467091

RESUMO

Nonaggregating HgTe colloidal quantum dots are synthesized without thiols as stabilizing ligands. The dots are spherical with a size tunability from 4.8 to 11.5 nm. When the results from optical and electrochemical measurement are combined, air-stable n-doping is observed in large sizes of HgTe quantum dots, which is attributed to the Hg-rich surface.

12.
ACS Nano ; 11(4): 4165-4173, 2017 04 25.
Artigo em Inglês | MEDLINE | ID: mdl-28314094

RESUMO

The absolute positions of the energy levels of colloidal quantum dots of Hg(S, Se, Te), which are of interest as mid-infrared materials, are determined by electrochemistry. The bulk valence bands are at -5.85, -5.50, and -4.77 eV (±0.05 eV) for zinc-blend HgS, HgSe, HgTe, respectively, in the same order as the anions p-orbital energies. The conduction bands are conversely at -5.20, -5.50, and -4.77 eV. The stable ambient n-doping of Hg(S, Se) quantum dots compared to HgTe arises because the conduction band is sufficiently lower than the measured environment Fermi level of ∼ -4.7 eV to allow for n-doping for HgS and HgSe quantum dots even with significant electron confinement. The position of the Fermi level and the quantum dots states are reported for a specific surface treatment with ethanedithiol and electrolyte environment. The positions are however sensitive to different surface treatments, providing an avenue to control doping. Electrochemical gating is further used to determine the carrier mobility in the films of the three different systems as a function of CQD size. HgSe and HgS show increasing mobility with increasing particle sizes while HgTe shows a nonmonotonous behavior, which is attributed to some degree of aggregation of HgTe QDs.

13.
ACS Nano ; 11(3): 2689-2696, 2017 03 28.
Artigo em Inglês | MEDLINE | ID: mdl-28195690

RESUMO

Colloidal perovskite nanocrystals support bright, narrow PL tunable over the visible spectrum. However, bandgap tuning of these materials remains limited to laboratory-scale syntheses. In this work, we present a polar-solvent-free ligand-mediated transport synthesis of high-quality organic-inorganic perovskite nanocrystals under ambient conditions with photoluminescence quantum yields up to 97%. Our synthesis employs a ligand-mediated transport mechanism that circumvents the need for exquisite external control (e.g., temperature control, inert-gas protection, dropwise addition of reagents) required by other methods due to extremely fast reaction kinetics. In the ligand-mediated transport mechanism, multiple equilibria cooperatively dictate reaction rates and enable precise control over NC size. These small nanocrystals exhibit high photoluminescence quantum yields due to quantum confinement. Nanosecond transient absorption spectroscopy experiments reveal a fluence-independent PL decay originating from exciton recombination. Two-dimensional electronic spectroscopy resolves multiple spectral features reflecting the electronic structure of the nanocrystals. The resolved features exhibit size-dependent spectral positions, further indicating the synthesized nanocrystals are quantum-confined.

14.
ACS Nano ; 10(2): 2121-7, 2016 Feb 23.
Artigo em Inglês | MEDLINE | ID: mdl-26820380

RESUMO

HgSe/CdS core/shell CQD are synthesized, and the changes in the optical absorption and luminescence are investigated. While HgSe quantum dots are naturally n-doped after synthesis, both as colloidal solutions and as films, the HgSe/CdS core/shell dots in solution lose the n-doping, as seen from the optical absorption in solution. However, n-doping is regained in films, and the intraband luminescence of the films of HgSe/CdS is greater than that of the cores. The shell also vastly improves the stability of the quantum dots films against sintering at 200 °C. After annealing at that temperature, the HgSe/CdS films retain a narrow intraband emission and sustain a higher laser power leading to brighter emission at 5 µm.

15.
ACS Nano ; 10(2): 2225-31, 2016 Feb 23.
Artigo em Inglês | MEDLINE | ID: mdl-26799582

RESUMO

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.

16.
ACS Nano ; 8(11): 11707-14, 2014 Nov 25.
Artigo em Inglês | MEDLINE | ID: mdl-25343383

RESUMO

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.

17.
ACS Nano ; 8(8): 8676-82, 2014 Aug 26.
Artigo em Inglês | MEDLINE | ID: mdl-25117471

RESUMO

HgTe colloidal quantum dots are synthesized with high monodispersivity with sizes up to ∼15 nm corresponding to a room temperature absorption edge at ∼5 µm. The shape is tetrahedral for larger sizes and up to five peaks are seen in the absorption spectra with a clear size dependence. The size range of the HgTe quantum dots is extended to ∼20 nm using regrowth. The corresponding room temperature photoluminescence and absorption edge reach into the long-wave infrared, past 8 µm. Upon cooling to liquid nitrogen temperature, a photoconductive response is obtained in the long-wave infrared region up to 12 µm. Configuration-interaction tight-binding calculations successfully explain the spectra and the size dependence. The five optical features can be assigned to sets of single hole to single electron transitions whose strengths are strongly influenced by the multiband/multiorbital character of the quantum-dot electronic states.

18.
J Phys Chem Lett ; 5(7): 1139-43, 2014 Apr 03.
Artigo em Inglês | MEDLINE | ID: mdl-26274461

RESUMO

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.

19.
ACS Nano ; 8(1): 283-91, 2014 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-24350673

RESUMO

Using electrochemical charge injection, the fluorescence lifetimes of negatively charged core/shell CdTe/CdSe QDs are measured as a function of core size and shell thickness. It is found that the ensemble negative trion lifetimes reach a maximum (∼4.5 ns) for an intermediate shell thickness. This leads to the smallest particles (∼4.5 nm) with the brightest trion to date. Single dot measurements show that the negative charge suppresses blinking and that the trion can be as bright as the exciton at room temperature. In contrast, the biexciton lifetimes remain short and exhibit only a monotonous increase with shell thickness, showing no correlation with the negative trion decays. The suppression of the Auger process in small negatively charged CdTe/CdSe quantum dots is unprecedented and a significant departure from prior results with ultrathick CdSe/CdS core/shell or dot-in-rod structures. The proposed reason for the optimum shell thickness is that the electron-hole overlap is restricted to the CdTe core while the electron is tuned to have zero kinetic energy in the core for that optimum shell thickness. The different trend of the biexciton lifetime is not explained but tentatively attributed to shorter-lived positive trions at smaller sizes. These results improve our understanding of multiexciton recombination in colloidal quantum dots and may lead to the design of bright charged QDs for more efficient light-emitting devices.

20.
Phys Rev Lett ; 111(24): 244502, 2013 Dec 13.
Artigo em Inglês | MEDLINE | ID: mdl-24483667

RESUMO

Newtonian fluid mechanics, in which the shear stress is proportional to the strain rate, is synonymous with the flow of simple liquids such as water. We report the measurement and theoretical verification of non-Newtonian, viscoelastic flow phenomena produced by the high-frequency (20 GHz) vibration of gold nanoparticles immersed in water-glycerol mixtures. The observed viscoelasticity is not due to molecular confinement, but is a bulk continuum effect arising from the short time scale of vibration. This represents the first direct mechanical measurement of the intrinsic viscoelastic properties of simple bulk liquids, and opens a new paradigm for understanding extremely high frequency fluid mechanics, nanoscale sensing technologies, and biophysical processes.

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