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1.
ACS Nano ; 2020 Jul 01.
Artigo em Inglês | MEDLINE | ID: mdl-32584026

RESUMO

Quasi-two-dimensional (2D) semiconductor nanoplatelets manifest strong quantum confinement with exceptional optical characteristics of narrow photoluminescence peaks with energies tunable by thickness with monolayer precision. We employed scanning tunneling spectroscopy (STS) in conjunction with optical measurements to probe the thickness-dependent band gap and density of excited states in a series of CdSe nanoplatelets. The tunneling spectra, measured in the double-barrier tunnel junction configuration, reveal the effect of quantum confinement on the band gap taking place mainly through a blue-shift of the conduction band edge, along with a signature of 2D electronic structure intermixed with finite lateral-size and/or defects effects. The STS fundamental band gaps are larger than the optical gaps as expected from the contributions of exciton binding in the absorption, as confirmed by theoretical calculations. The calculations also point to strong valence band mixing between the light- and split-off hole levels. Strikingly, the energy difference between the heavy-hole and light-hole levels in the tunneling spectra are significantly larger than the corresponding values extracted from the absorption spectra. Possible explanations for this, including an interplay of nanoplatelet charging, dielectric confinement, and difference in exciton binding energy for light and heavy holes, are analyzed and discussed.

2.
Phys Chem Chem Phys ; 20(37): 24444-24452, 2018 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-30221290

RESUMO

In-gap states in solar cell absorbers that are recombination centers determine the cell's photovoltaic performance. Using scanning tunneling spectroscopy (STS), temperature-dependent photoconductivity and steady-state photocarrier-grating measurements we probed, directly and indirectly, the energies of such states, both at the surface and in the bulk of two similar, but different halide perovskites, the single cation MAPbI3 (here MAPI) and the mixed cation halide perovskite, FA0.79MA0.16Cs0.05Pb(I0.83Br0.17)3 (here MCHP). We found a correlation between the energy distribution of the in-gap states, as determined by STS measurements, and their manifestation in the photo-transport parameters of the MCHP absorbers. In particular, our results suggest that the in-gap recombination centers in the MCHP are shallower than those of MAPI. This can be one explanation for the better photovoltaic efficiency of the former.

3.
Nanotechnology ; 29(35): 355705, 2018 Aug 31.
Artigo em Inglês | MEDLINE | ID: mdl-29862985

RESUMO

Developing new methods, other than size and shape, for controlling the optoelectronic properties of semiconductor nanocrystals is a highly desired target. Here we demonstrate that the photoluminescence (PL) of silicon nanocrystals (SiNCs) can be tuned in the range 685-800 nm solely via surface functionalization with alkynyl(aryl) (phenylacetylene, 2-ethynylnaphthalene, 2-ethynyl-5-hexylthiophene) surface groups. Scanning tunneling microscopy/spectroscopy on single nanocrystals revealed the formation of new in-gap states adjacent to the conduction band edge of the functionalized SiNCs. PL red-shifts were attributed to emission through these in-gap states, which reduce the effective band gap for the electron-hole recombination process. The observed in-gap states can be associated with new interface states formed via (-Si-C≡C-) bonds in combination with conjugated molecules as indicated by ab initio calculations. In contrast to alkynyl(aryl)s, the formation of in-gap states and shifts in PL maximum of the SiNCs were not observed with aryl (phenyl, naphthalene, 2-hexylthiophene) and alkynyl (1-dodecyne) surface groups. These outcomes show that surface functionalization with alkynyl(aryl) molecules is a valuable tool to control the electronic structure and optical properties of SiNCs via tuneable interface states, which may enhance the performance of SiNCs in semiconductor devices.

4.
Nanoscale ; 9(45): 17884-17892, 2017 Nov 23.
Artigo em Inglês | MEDLINE | ID: mdl-29120002

RESUMO

The electrical and optical properties of semiconductor nanocrystals (NCs) can be controlled, in addition to size and shape, by doping. However, such a process is not trivial in NCs due to the high formation energy of dopants there. Nevertheless, it has been shown theoretically that in the case of B and P (acceptor/donor) codoped Si-NCs the formation energy is reduced relative to that of single type doping. Previous comprehensive measurements on ensembles of such codoped Si-NCs have pointed to the presence of donor and acceptor states within the energy gap. However, such a conjecture has not been directly verified previously. Following that, we investigate here the electronic properties of B and P codoped Si-NCs via Scanning Tunneling Spectroscopy. We monitored the quantum confinement effect in this system, for which the energy gap changed from ∼1.4 eV to ∼1.8 eV with the decrease of NC diameter from 8.5 to 3.5 nm. Importantly, all spectra showed two in-gap band-states, one close to the conduction band edge and the other to the valence band edge, which we attribute to the P and B dopant levels, respectively. The energy separation between these dopants states decrease monotonically with increasing NC diameter, in parallel to the decrease of the conduction-to-valence bands separation. A fundamental quantity that is derived directly for these Si-NCs is the intrinsic like position of the Fermi energy, a non-trivial result that is very relevant for understanding the system. Following the above results we suggest an explanation for the character and the origin of the dopants bands.

5.
J Phys Chem Lett ; 7(24): 5219-5226, 2016 Dec 15.
Artigo em Inglês | MEDLINE | ID: mdl-27973905

RESUMO

Photovoltaic solar cells operate under steady-state conditions that are established during the charge carrier excitation and recombination. However, to date no model of the steady-state recombination scenario in halide perovskites has been proposed. In this Letter we present such a model that is based on a single type of recombination center, which is deduced from our measurements of the illumination intensity dependence of the photoconductivity and the ambipolar diffusion length in those materials. The relation between the present results and those from time-resolved measurements, such as photoluminescence that are commonly reported in the literature, is discussed.

6.
Angew Chem Int Ed Engl ; 55(26): 7393-7, 2016 06 20.
Artigo em Inglês | MEDLINE | ID: mdl-27144670

RESUMO

Hybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3-hexylthiophene) (SiNC@P3HT) by applying surface-initiated Kumada catalyst transfer polycondensation (SI-KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor-acceptor interfaces. Systematic characterization provides evidence of a core-shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type-II heterostructure.

7.
Nanoscale ; 8(15): 7849-53, 2016 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-27020915

RESUMO

Optoelectronic properties of Si nanocrystals (SiNCs) were studied by combining scanning tunneling spectroscopy (STS) and optical measurements. The photoluminescence (PL) of phenylacetylene functionalized SiNCs red shifts relative to hexyl- and phenyl-capped counterparts, whereas the absorption spectra and the band gaps extracted from STS are similar for all surface groups. However, an in-gap state near the conduction band edge was detected by STS only for the phenylacetylene terminated SiNCs, which can account for the PL shift via relaxation across this state.

8.
J Phys Chem Lett ; 6(17): 3396-402, 2015 Sep 03.
Artigo em Inglês | MEDLINE | ID: mdl-26275992

RESUMO

To address the yet open question regarding the nature of quantum confinement in Ge nanocrystals (Ge NCs) we employed scanning tunneling spectroscopy to monitor the electronic structure of individual isolated Ge NCs as a function of their size. The (single-particle) band gaps extracted from the tunneling spectra increase monotonically with decreasing nanocrystal size, irrespective of the capping ligands, manifesting the effect of quantum confinement. Band-gap widening of ∼1 eV with respect to the bulk value was observed for Ge-NCs 3 nm in diameter. The picture emerging from comparison with theoretical calculations and other experimental results is discussed.


Assuntos
Geranium/química , Nanopartículas/química , Teoria Quântica
9.
Phys Rev Lett ; 108(7): 076603, 2012 Feb 17.
Artigo em Inglês | MEDLINE | ID: mdl-22401233

RESUMO

We investigated the electronic properties around grain boundaries of polycrystalline Cu(In(1-x),Ga(x)Se(2)) films as a function of Ga content, using scanning tunneling spectroscopy. Spectra acquired on samples with low Ga content (x=0 and 0.33) reveal downward band bending with respect to adjacent p-type grains, suggesting type inversion at the surface of grain boundaries. Such a behavior was not observed for samples with high Ga contents. These results are consistent with our atomic force microscopy data and may shed light on the origin of the x-dependent efficiency for polycrystalline Cu(In(1-x),Ga(x)Se(2))-based solar cells.

10.
J Colloid Interface Sci ; 364(2): 379-87, 2011 Dec 15.
Artigo em Inglês | MEDLINE | ID: mdl-21958442

RESUMO

The potential of reverse hexagonal mesophases based on monoolein (GMO) and glycerol (as cosolvent) to facilitate the solubilization of proteins, such as insulin was explored. H(II) mesophases composed of GMO/decane/water were compared to GMO/decane/glycerol/water and GMO/phosphatidylcholine (PC)/decane/glycerol/water systems. The stability of insulin was tested, applying external physical modifications such as low pH and heat treatment (up to 70°C), in which insulin is known to form ordered amyloid-like aggregates (that are associated with several neurodegenerative diseases) with a characteristic cross ß-pleated sheet structure. The impact of insulin confinement within these carriers on its stability, unfolding, and aggregation pathways was studied by combining SAXS, FTIR, and AFM techniques. These techniques provided a better insight into the molecular level of the "component interplay" in solubilizing and stabilizing insulin and its conformational modifications that dictate its final aggregate morphology. PC enlarged the water channels while glycerol shrank them, yet both facilitated insulin solubilization within the channels. The presence of glycerol within the mesophase water channels led to the formation of stronger hydrogen bonds with the hosting medium that enhanced the thermal stability of the protein and remarkably affected the unfolding process even after heat treatment (at 70°C for 60 min).


Assuntos
Glicerol/química , Insulina/química , Fosfatidilcolinas/química , Microscopia de Força Atômica , Solubilidade , Espectroscopia de Infravermelho com Transformada de Fourier
11.
ACS Nano ; 3(11): 3537-42, 2009 Nov 24.
Artigo em Inglês | MEDLINE | ID: mdl-19928933

RESUMO

We report here a concept for utilization of the "coffee ring effect" and inkjet printing to obtain transparent conductive patterns, which can replace the widely used transparent conductive oxides, such as ITO. The transparent conductive coating is achieved by forming a 2-D array of interconnected metallic rings. The rim of the individual rings is less than 10 microm in width and less than 300 nm in height, surrounding a "hole" with a diameter of about 150 microm; therefore the whole array of the interconnected rings is almost invisible to the naked eye. The rims of the rings are composed of self-assembled, closely packed silver nanoparticles, which make the individual rings and the resulting array electrically conductive. The resulting arrays of rings have a transparency of 95%; resistivity of 0.5 cm(2) was 4 +/- 0.5 Omega/, which is better than conventional ITO transparent thin films. The silver rings and arrays are fabricated by a very simple, low cost process, based on inkjet printing of a dispersion of 0.5 wt % silver nanoparticles (approximately 20 nm diameter) on plastic substrates. The performance of this transparent conductive coating was demonstrated by using it as an electrode for a plastic electroluminescent device, demonstrating the applicability of this concept in plastics electronics. It is expected that such transparent conductive coatings can be used in a wide range of applications such as displays (LCD, plasma, touch screens, e-paper), lighting devices (electroluminescence, OLED), and solar cells.

12.
Nanotechnology ; 19(6): 065201, 2008 Feb 13.
Artigo em Inglês | MEDLINE | ID: mdl-21730694

RESUMO

We studied the electronic level structure of assemblies of InAs quantum dots and CdSe nanorods cross-linked by 1,4-phenylenediamine molecules using scanning tunneling spectroscopy. We found that the bandgap in these arrays is reduced with respect to the corresponding ligand-capped nanocrystal arrays. In addition, a pronounced sub-gap spectral structure commonly appeared which can be attributed to unpassivated nanocrystal surface states or associated with linker-molecule-related levels. The exchange of the ligands by the linker molecules also affected the structural array properties. Most significantly, clusters of close-packed standing CdSe nanorods were formed.

13.
J Am Chem Soc ; 129(41): 12549-62, 2007 Oct 17.
Artigo em Inglês | MEDLINE | ID: mdl-17887674

RESUMO

IF-Mo1-xNbxS2 nanoparticles have been synthesized by a vapor-phase reaction involving the respective metal halides with H2S. The IF-Mo1-xNbxS2 nanoparticles, containing up to 25% Nb, were characterized by a variety of experimental techniques. Analysis of the powder X-ray powder diffraction, X-ray photoelectron spectroscopy, and different electron microscopy techniques shows that the majority of the Nb atoms are organized as nanosheets of NbS2 within the MoS2 host lattice. Most of the remaining Nb atoms (3%) are interspersed individually and randomly in the MoS2 host lattice. Very few Nb atoms, if any, are intercalated between the MoS2 layers. A sub-nanometer film of niobium oxide seems to encoat the majority of the nanoparticles. X-ray photoelectron spectroscopy in the chemically resolved electrical measurement mode (CREM) and scanning probe microscopy measurements of individual nanoparticles show that the mixed IF nanoparticles are metallic independent of the substitution pattern of the Nb atoms in the lattice of MoS2 (whereas unsubstituted IF-MoS2 nanoparticles are semiconducting). Furthermore the IF-Mo1-xNbxS2 nanoparticles are found to exhibit interesting single electron tunneling effects at low temperatures.

14.
Nano Lett ; 6(4): 760-4, 2006 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-16608279

RESUMO

Current-voltage characteristics measured using STM on fullerene-like WS2 nanoparticles show zero-bias current and contain segments in which the tunneling current flows opposite to the applied bias voltage. In addition, negative differential conductance peaks emerge in these reversed current segments, and the characteristics are hysteretic with respect to the change in the voltage sweep direction. Such unusual features resemble those appearing in cyclic voltammograms, but are uniquely observed here in tunneling spectra measured in vacuum, as well as in ambient and dry atmosphere conditions. This behavior is attributed to tunneling-driven electrochemical processes.


Assuntos
Eletroquímica/métodos , Fulerenos/química , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Compostos de Enxofre/química , Compostos de Tungstênio/química , Condutividade Elétrica , Fulerenos/análise , Teste de Materiais , Microscopia de Tunelamento/métodos , Tamanho da Partícula , Semicondutores , Análise Espectral
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