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1.
ACS Nano ; 12(10): 10008-10015, 2018 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-30226751

RESUMO

The optoelectronic properties of semiconductor nanocrystals (NCs) have led to efforts to integrate them as the active material in light-emitting diodes, solid-state lighting, and lasers. Understanding related high carrier injection conditions is therefore critical as resultant thermal effects can impact optical properties. The physical integrity of NCs is indeed questionable as recent transient X-ray diffraction studies have suggested that nanoscopic particles reversibly lose crystalline order, or melt, under high fluence photoexcitation. Informed by such studies, here, we examine CdSe NCs under elevated fluences to determine the impact of lattice disordering on optical properties. To this end, we implement intensity-dependent transient absorption using both one- and two-pump methods where the latter effectively subtracts out the NC optical signatures associated with lower fluence photoexcitation, especially band-edge features. At elevated fluences, we observe a long-lived induced absorption at a lower energy than the crystalline-NC bandgap across a wide range of sizes that follows power-dependent trends and kinetics consistent with the prior transient X-ray measurements. NC photoluminescence studies provide further evidence that melting influences optical properties. These methods of characterizing bandgap narrowing caused by lattice disordering could facilitate routes to improved optical amplification and band-edge emission at high excitation density.

2.
J Athl Train ; 53(6): 590-596, 2018 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-29975572

RESUMO

CONTEXT: Knowledge of the bilateral difference in humeral torsion (HT) enables clinicians to implement appropriate interventions for soft tissue restrictions of the shoulder to restore rotational motion and reduce injury risk. Whereas the current ultrasound method for measuring HT requires 2 assessors, a more efficient 1-person technique (1PT) may be of value. OBJECTIVE: To determine if a 1PT is a reliable and valid alternative to the established 2-person technique (2PT) for indirectly measuring HT using ultrasound. DESIGN: Descriptive laboratory study. SETTING: Biomechanics laboratory. PATIENTS OR OTHER PARTICIPANTS: A convenience sample of 16 volunteers (7 men, 9 women; age = 26.9 ± 6.8 years, height = 172.2 ± 10.7 cm, mass = 80.0 ± 13.3 kg). MAIN OUTCOME MEASURE(S): We collected the HT data using both the 1PT and 2PT from a total of 30 upper extremities (16 left, 14 right). Within-session intrarater reliability (intraclass correlation coefficient; ICC [3,1]) and standard error of measurement (SEM) were assessed for both techniques. Simple linear regression and Bland-Altman analysis were used to examine the validity of the 1PT when compared with the established 2PT. RESULTS: The 1PT (ICC [3,1] = 0.992, SEM = 0.8°) and 2PT (ICC [3,1] = 0.979, SEM = 1.1°) demonstrated excellent within-session intrarater reliability. A strong linear relationship was demonstrated between the HT measurements collected with both techniques ( r = 0.963, r2 = 0.928, F1,28 = 361.753, P < .001). A bias of -1.2° ± 2.6° was revealed, and the 95% limits of agreement indicated the 2 techniques can be expected to vary from -6.3° to 3.8°. CONCLUSIONS: The 1PT for measuring HT using ultrasound was a reliable and valid alternative to the 2PT. By reducing the number of testers involved, the 1PT may provide clinicians with a more efficient and practical means of obtaining these valuable clinical data. a.


Assuntos
Fraturas Ósseas/prevenção & controle , Úmero , Lesões do Ombro/prevenção & controle , Articulação do Ombro , Ultrassonografia/métodos , Adulto , Feminino , Fraturas Ósseas/etiologia , Fraturas Ósseas/fisiopatologia , Humanos , Úmero/diagnóstico por imagem , Úmero/fisiopatologia , Modelos Lineares , Masculino , Amplitude de Movimento Articular , Reprodutibilidade dos Testes , Ombro/diagnóstico por imagem , Ombro/fisiopatologia , Lesões do Ombro/etiologia , Lesões do Ombro/fisiopatologia , Articulação do Ombro/diagnóstico por imagem , Articulação do Ombro/fisiopatologia
3.
J Phys Chem Lett ; 9(16): 4481-4487, 2018 Aug 16.
Artigo em Inglês | MEDLINE | ID: mdl-30011208

RESUMO

We report femtosecond stimulated Raman spectroscopy (FSRS) measurements on dispersions of CdSe semiconductor nanocrystals (NCs) as a function of particle size and pump fluence. Upon photoexcitation, we observe depletion of stimulated Raman gain corresponding to generation of longitudinal optical (LO) phonons followed by recovery on picosecond timescales. At higher fluences, production of multiple excitons slows recovery of FSRS signals, which we attribute to sustained increases of LO phonon populations due to multiexcitonic Auger heating. Owing to the discretized electronic structure of these NCs, such heating cannot be readily monitored via electronic spectroscopic analysis of high-energy band tails as has been performed for higher-dimensional materials. Notably, recovery timescales exceed those of the biexcitonic Auger recombination process and as such reveal overall thermalization timescales likely owing to an acoustic phonon thermalization bottleneck that dictates the cooling timescale.

4.
J Phys Chem Lett ; 9(3): 628-634, 2018 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-29320200

RESUMO

We report on a scheme for estimating intercalant jump-diffusion barriers that are typically obtained from demanding density functional theory-nudged elastic band calculations. The key idea is to relax a chain of states in the field of the electrostatic potential that is averaged over a spherical volume using different finite-size ion models. For magnesium migrating in typical intercalation materials such as transition-metal oxides, we find that the optimal model is a relatively large shell. This data-driven result parallels typical assumptions made in models based on Onsager's reaction field theory to quantitatively estimate electrostatic solvent effects. Because of its efficiency, our potential of electrostatics-finite ion size (PfEFIS) barrier estimation scheme will enable rapid identification of materials with good ionic mobility.

5.
Chem Commun (Camb) ; 53(57): 7998-8001, 2017 Jul 13.
Artigo em Inglês | MEDLINE | ID: mdl-28664208

RESUMO

In this work, we identify a new potential Mg battery cathode structure Mo3(PO4)3O, which is predicted to exhibit ultra-fast Mg2+ diffusion and relatively high voltage based on first-principles density functional theory calculations. Nudged elastic band calculations reveal that the migration barrier of the percolation channel is only ∼80 meV, which is remarkably low, and comparable to the best Li-ion conductors. This low barrier is verified by ab initio molecular dynamics and kinetic Monte Carlo simulations. The voltage and specific energy are predicted to be ∼1.98 V and ∼173 W h kg-1, respectively. If confirmed by experiments, this material would have the highest known Mg mobility among inorganic compounds.

6.
Nano Lett ; 17(9): 5314-5320, 2017 09 13.
Artigo em Inglês | MEDLINE | ID: mdl-28753318

RESUMO

Ultrafast optical pump, X-ray diffraction probe experiments were performed on CdSe nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump fluence. Bragg peak shifts related to heating and peak amplitude reduction associated with lattice disordering are observed. For smaller NCs, melting initiates upon absorption of as few as ∼15 electron-hole pair excitations per NC on average (0.89 excitations/nm3 for a 1.5 nm radius) with roughly the same excitation density inducing melting for all examined NCs. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. Zincblende and wurtzite NCs revert to initial structures following intense photoexcitation suggesting melting occurs primarily at the surface, as supported by simulations. Electronic structure calculations relate significant band gap narrowing with decreased crystallinity. These findings reflect the need to consider the physical stability of nanomaterials and related electronic impacts in high intensity excitation applications such as lasing and solid-state lighting.

7.
Int J Sports Phys Ther ; 12(3): 305-313, 2017 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-28593084

RESUMO

BACKGROUND: Muscular weakness of the shoulder complex is commonly found in patients presenting with scapular dyskinesis; however, little is known regarding muscular performance in healthy individuals with scapular dyskinesis. PURPOSE: To compare isometric strength measures of the shoulder complex between healthy individuals with and without scapular dyskinesis. It was hypothesized that healthy individuals with scapular dyskinesis would demonstrate decreased isometric strength of the scapular stabilizers and rotator cuff when compared to healthy individuals without scapular dyskinesis. STUDY DESIGN: Cross-sectional study. METHODS: Forty healthy, college-aged participants were recruited. Sixty-eight percent of subjects (27 of 40) presented with scapular dyskinesis. Thus, a matched-pairs analysis was conducted with 26 subjects (age: 22.00 ± 2.06 y; height: 168.77 ± 8.07 cm; mass: 70.98 ± 13.14 kg; BMI: 24.75 ± 3.04 kg/m2; 6 males; 20 females). The presence of scapular dyskinesis was determined visually using the scapular dyskinesis test with a dichotomous outcome (yes/no). Strength of the scapular stabilizers and rotator cuff was assessed via manual muscle testing using a handheld dynamometer. Force measures obtained with the handheld dynamometer were used to quantify strength. For each muscle tested, the mean peak force of three trials were normalized to body weight and used for data analysis. Additionally, strength ratios were calculated and analyzed. Differences in strength and strength ratios between those with and without scapular dyskinesis were compared using separate two-way mixed ANOVAs with repeated measures. RESULTS: No significant differences for either strength (F1.83,43.92 = 1.10, p = .34) or strength ratios (F1.83,44.02 = 1.93, p = .16) were observed between those with and without scapular dyskinesis. A significant main effect (F1.83,43.92 = 239.32, p < .01) for muscles tested was observed, and post-hoc analysis revealed significant trends resulting in a generalized order: the upper trapezius generated the greatest amount of force, followed by serratus anterior and middle trapezius, lower trapezius, supraspinatus, medial rotators, and lateral rotators. CONCLUSION: The results of this study indicate that differences in shoulder muscle strength do not exist between healthy subjects with and without scapular dyskinesis. Additionally, scapular dyskinesis appears to be prevalent in healthy populations. LEVEL OF EVIDENCE: Level 3.

8.
Chem Commun (Camb) ; 53(37): 5171-5174, 2017 May 04.
Artigo em Inglês | MEDLINE | ID: mdl-28439589

RESUMO

We propose that Ti-containing post-spinels may offer a practically-accessible route to fast multivalent ion diffusion in close-packed oxide lattices, with the caveat that substantial thermodynamic driving forces for conversion reactions exist.

9.
Chem Rev ; 117(5): 4287-4341, 2017 Mar 08.
Artigo em Inglês | MEDLINE | ID: mdl-28269988

RESUMO

The rapidly expanding field of nonaqueous multivalent intercalation batteries offers a promising way to overcome safety, cost, and energy density limitations of state-of-the-art Li-ion battery technology. We present a critical and rigorous analysis of the increasing volume of multivalent battery research, focusing on a wide range of intercalation cathode materials and the mechanisms of multivalent ion insertion and migration within those frameworks. The present analysis covers a wide variety of material chemistries, including chalcogenides, oxides, and polyanions, highlighting merits and challenges of each class of materials as multivalent cathodes. The review underscores the overlap of experiments and theory, ranging from charting the design metrics useful for developing the next generation of MV-cathodes to targeted in-depth studies rationalizing complex experimental results. On the basis of our critical review of the literature, we provide suggestions for future multivalent cathode studies, including a strong emphasis on the unambiguous characterization of the intercalation mechanisms.

10.
World J Orthop ; 6(10): 783-94, 2015 Nov 18.
Artigo em Inglês | MEDLINE | ID: mdl-26601060

RESUMO

The shoulder complex presents unique challenges for measuring motion as the scapula, unlike any other bony segment in the body, glides and rotates underneath layers of soft tissue and skin. The ability for clinicians and researchers to collect meaningful kinematic data is dependent on the reliability and validity of the instrumentation utilized. The aim of this study was to review the relevant literature pertaining to the reliability and validity of electromagnetic tracking systems (ETS) and digital inclinometers for assessing shoulder complex motion. Advances in technology have led to the development of biomechanical instrumentation, like ETS, that allow for the collection of three-dimensional kinematic data. The existing evidence has demonstrated that ETS are reliable and valid instruments for collecting static and dynamic kinematic data of the shoulder complex. Similarly, digital inclinometers have become increasingly popular among clinicians due to their cost effectiveness and practical use in the clinical setting. The existing evidence supports the use of digital inclinometers for the collection of shoulder complex kinematics as these instruments have been demonstrated to yield acceptable reliability and validity. While digital inclinometers pose a disadvantage to ETS regarding accuracy, precision, and are limited to two-dimensional and static measurements, this instrument provides clinically meaningful data that allow clinicians and researchers the ability to measure, monitor, and compare shoulder complex kinematics.

11.
Nano Lett ; 15(10): 7161-7, 2015 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-26397120

RESUMO

Botulinum neurotoxin (BoNT) presents a significant hazard under numerous realistic scenarios. The standard detection scheme for this fast-acting toxin is a lab-based mouse lethality assay that is sensitive and specific, but slow (∼2 days) and requires expert administration. As such, numerous efforts have aimed to decrease analysis time and reduce complexity. Here, we describe a sensitive ratiometric fluorescence resonance energy transfer scheme that utilizes highly photostable semiconductor quantum dot (QD) energy donors and chromophore conjugation to compact, single chain variable antibody fragments (scFvs) to yield a fast, fieldable sensor for BoNT with a 20-40 pM detection limit, toxin quantification, adjustable dynamic range, sensitivity in the presence of interferents, and sensing times as fast as 5 min. Through a combination of mutations, we achieve stabilized scFv denaturation temperatures of more than 60 °C, which bolsters fieldability. We also describe adaptation of the assay into a microarray format that offers persistent monitoring, reuse, and multiplexing.


Assuntos
Toxinas Botulínicas/análise , Pontos Quânticos , Radiometria/métodos , Anticorpos de Cadeia Única/química , Transferência Ressonante de Energia de Fluorescência , Limite de Detecção
12.
ACS Nano ; 9(6): 6278-87, 2015 Jun 23.
Artigo em Inglês | MEDLINE | ID: mdl-26020654

RESUMO

We examine the role played by surface structure and passivation in thermal transport at semiconductor/organic interfaces. Such interfaces dominate thermal transport in semiconductor nanomaterials owing to material dimensions much smaller than the bulk phonon mean free path. Utilizing reverse nonequilibrium molecular dynamics simulations, we calculate the interfacial thermal conductance (G) between a hexane solvent and chemically passivated wurtzite CdSe surfaces. In particular, we examine the dependence of G on the CdSe slab thickness, the particular exposed crystal facet, and the extent of surface passivation. Our results indicate a nonmonotonic dependence of G on ligand-grafting density, with interfaces generally exhibiting higher thermal conductance for increasing surface coverage up to ∼0.08 ligands/Å(2) (75-100% of a monolayer, depending on the particular exposed facet) and decreasing for still higher coverages. By analyzing orientational ordering and solvent penetration into the ligand layer, we show that a balance of competing effects is responsible for this nonmonotonic dependence. Although the various unpassivated CdSe surfaces exhibit similar G values, the crystal structure of an exposed facet nevertheless plays an important role in determining the interfacial thermal conductance of passivated surfaces, as the density of binding sites on a surface determines the ligand-grafting densities that may ultimately be achieved. We demonstrate that surface passivation can increase G relative to a bare surface by roughly 1 order of magnitude and that, for a given extent of passivation, thermal conductance can vary by up to a factor of ∼2 between different surfaces, suggesting that appropriately tailored nanostructures may direct heat flow in an anisotropic fashion for interface-limited thermal transport.

13.
ACS Nano ; 8(9): 9219-23, 2014 Sep 23.
Artigo em Inglês | MEDLINE | ID: mdl-25181589

RESUMO

We report the photoluminescence (PL) properties of colloidal Si nanocrystals (NCs) up to 800 K and observe PL retention on par with core/shell structures of other compositions. These alkane-terminated Si NCs even emit at temperatures well above previously reported melting points for oxide-embedded particles. Using selected area electron diffraction (SAED), powder X-ray diffraction (XRD), liquid drop theory, and molecular dynamics (MD) simulations, we show that melting does not play a role at the temperatures explored experimentally in PL, and we observe a phase change to ß-SiC in the presence of an electron beam. Loss of diffraction peaks (melting) with recovery of diamond-phase silicon upon cooling is observed under inert atmosphere by XRD. We further show that surface passivation by covalently bound ligands endures the experimental temperatures. These findings point to covalently bound organic ligands as a route to the development of NCs for use in high temperature applications, including concentrated solar cells and electrical lighting.

14.
ACS Nano ; 8(1): 977-85, 2014 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-24328364

RESUMO

We examine the stability of excitons in quantum-confined InP nanocrystals as a function of temperature elevation up to 800 K. Through the use of static and time-resolved spectroscopy, we find that small inorganic capping ligands substantially improve the temperature dependent photoluminescence quantum yield relative to native organic ligands and perform similarly to a wide band gap inorganic shell. For this composition, we identify the primary exciton loss mechanism as electron trapping through a combination of transient absorption and transient photoluminescence measurements. Density functional theory indicates little impact of studied inorganic ligands on InP core states, suggesting that reduced thermal degradation relative to organic ligands yields improved stability; this is further supported by a lack of size dependence in photoluminescence quenching, pointing to the dominance of surface processes, and by relative thermal stabilities of the surface passivating media. Thus, small inorganic ligands, which benefit device applications due to improved carrier access, also improve the electronic integrity of the material during elevated temperature operation and subsequent to high temperature material processing.

15.
Phys Chem Chem Phys ; 15(20): 7441-9, 2013 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-23604217

RESUMO

This perspective describes the influence of interfacial charge transfer-type interactions on the optical spectra and hot electron cooling processes in plasmonic nanoparticles (NPs), and ongoing work to optimize these interactions for charge extraction from the plasmon or hot electron state. The manuscript focuses on interfaces of metal NPs with organic molecules and with semiconductors. Charge extraction from multi-electron excited states has applications in photodetection, sensing, and conversion of solar energy to electricity and fuels.

16.
Proc Natl Acad Sci U S A ; 110(11): 4212-7, 2013 Mar 12.
Artigo em Inglês | MEDLINE | ID: mdl-23440215

RESUMO

This paper describes measurements of the dynamics of hot electron cooling in photoexcited gold nanoparticles (Au NPs) with diameters of ∼3.5 nm, and passivated with either a hexadecylamine or hexadecanethiolate adlayer, using ultrafast transient absorption spectroscopy. Fits of these dynamics with temperature-dependent Mie theory reveal that both the electronic heat capacity and the electron-phonon coupling constant are larger for the thiolated NPs than for the aminated NPs, by 40% and 30%, respectively. Density functional theory calculations on ligand-functionalized Au slabs show that the increase in these quantities is due to an increased electronic density of states near the Fermi level upon ligand exchange from amines to thiolates. The lifetime of hot electrons, which have thermalized from the initial plasmon excitation, increases with increasing electronic heat capacity, but decreases with increasing electron-phonon coupling, so the effects of changing surface chemistry on these two quantities partially cancel to yield a hot electron lifetime of thiolated NPs that is only 20% longer than that of aminated NPs. This analysis also reveals that incorporation of a temperature-dependent electron-phonon coupling constant is necessary to adequately fit the dynamics of electron cooling.


Assuntos
Elétrons , Ouro/química , Nanopartículas Metálicas/química , Modelos Químicos , Tamanho da Partícula
17.
Phys Rev Lett ; 111(10): 107401, 2013 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-25166708

RESUMO

We report femtosecond stimulated Raman spectroscopy measurements of lattice dynamics in semiconductor nanocrystals and characterize longitudinal optical (LO) phonon production during confinement-enhanced, ultrafast intraband relaxation. Stimulated Raman signals from unexcited CdSe nanocrystals produce a spectral shape similar to spontaneous Raman signals. Upon photoexcitation, stimulated Raman amplitude decreases owing to experimentally resolved ultrafast phonon generation rates within the lattice. We find a ∼600 fs, particle-size-independent depletion time attributed to hole cooling, evidence of LO-to-acoustic down-conversion, and LO phonon mode softening.

18.
Nano Lett ; 12(11): 5797-801, 2012 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-23066718

RESUMO

Known manipulations of semiconductor thermal transport properties rely upon higher-order material organization. Here, using time-resolved optical signatures of phonon transport, we demonstrate a "bottom-up" means of controlling thermal outflow in matrix-embedded semiconductor nanocrystals. Growth of an electronically noninteracting ZnS shell on a CdSe core modifies thermalization times by an amount proportional to the overall particle radius. Using this approach, we obtain changes in effective thermal conductivity of up to 5× for a nearly constant energy gap.


Assuntos
Nanopartículas/química , Nanotecnologia/métodos , Pontos Quânticos , Semicondutores , Compostos de Cádmio/química , Difusão , Temperatura Alta , Tamanho da Partícula , Fônons , Compostos de Selênio/química , Espectrofotometria/métodos , Sulfetos/química , Temperatura , Condutividade Térmica , Compostos de Zinco/química
19.
Nano Lett ; 12(11): 5545-51, 2012 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-23030497

RESUMO

Nonblinking excitonic emission from near-infrared and type-II nanocrystal quantum dots (NQDs) is reported for the first time. To realize this unusual degree of stability at the single-dot level, novel InP/CdS core/shell NQDs were synthesized for a range of shell thicknesses (~1-11 monolayers of CdS). Ensemble spectroscopy measurements (photoluminescence peak position and radiative lifetimes) and electronic structure calculations established the transition from type-I to type-II band alignment in these heterostructured NQDs. More significantly, single-NQD studies revealed clear evidence for blinking suppression that was not strongly shell-thickness dependent, while photobleaching and biexciton lifetimes trended explicitly with extent of shelling. Specifically, very long biexciton lifetimes-up to >7 ns-were obtained for the thickest-shell structures, indicating dramatic suppression of nonradiative Auger recombination. This new system demonstrates that electronic structure and shell thickness can be employed together to effect control over key single-dot and ensemble NQD photophysical properties.


Assuntos
Pontos Quânticos , Espectroscopia de Luz Próxima ao Infravermelho/métodos , Compostos de Cádmio/química , Eletrônica , Elétrons , Índio/química , Microscopia Eletrônica de Transmissão/métodos , Nanopartículas , Nanotecnologia/métodos , Paládio/química , Tamanho da Partícula , Fosfatos/química , Fotoquímica/métodos , Espectrometria de Fluorescência/métodos , Sulfetos/química , Temperatura , Fatores de Tempo
20.
Nano Lett ; 12(8): 4200-5, 2012 Aug 08.
Artigo em Inglês | MEDLINE | ID: mdl-22757779

RESUMO

A lack of consensus persists regarding the origin of photoluminescence in silicon nanocrystals. Here we report pressure-dependences of X-ray diffraction and photoluminescence from alkane-terminated colloidal particles. We determine the diamond-phase bulk modulus, observe multiple phase transitions, and importantly find a systematic photoluminescence red shift that matches the X(conduction)-to-Γ(valence) transition of bulk crystalline silicon. These results, reinforced by calculations, suggest that the efficient photoluminescence, frequently attributed to defects, arises instead from core-states that remain highly indirect despite quantum confinement.

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