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1.
Nano Lett ; 20(2): 1023-1032, 2020 Feb 12.
Artigo em Inglês | MEDLINE | ID: mdl-31917588

RESUMO

Metallic halide perovskites are promising for low-cost, low-consumption, flexible optoelectronic devices. However, research is lacking on light propagation and dielectric behaviors as fundamental properties for optoelectronic perovskite applications, particularly the mechanism supporting a strong light-matter interaction and the different properties of low-dimensional structures from their bulk counterparts. We use spatially resolved photoluminescence (SRPL) spectroscopy to explore light propagation and measure the refractive index of CsPbBr3 nanowires (NWs). Owing to strong exciton-photon interactions, light is guided as an exciton-polariton inside the NWs at room temperature. Remarkable spatial dispersion is confirmed, in which both the real and imaginary parts of the refractive index increase dramatically approaching exciton resonance, thus slowing light and enhancing absorption, respectively. Reducing the NWs dimension increases exciton-photon coupling and the exciton fraction, increasing the light absorption coefficient and group index 5- and 3-fold, respectively, relative to those of bulk films and slowing the light group velocity by ∼74%. Furthermore, dispersive absorption induces an energy redshift to the propagating PL at 4.1-5.5 meV µm-1 until the bottleneck region. These findings clarify light-matter interaction in confined perovskite structures to improve their optoelectronic device performance.

2.
Adv Mater ; 32(4): e1905896, 2020 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-31825536

RESUMO

2D magnetic materials have attracted intense attention as ideal platforms for constructing multifunctional electronic and spintronic devices. However, most of the reported 2D magnetic materials are mainly achieved by the mechanical exfoliation route. The direct synthesis of such materials is still rarely reported, especially toward thickness-controlled synthesis down to the 2D limit. Herein, the thickness-tunable synthesis of nanothick rhombohedral Cr2 S3 flakes (from ≈1.9 nm to tens of nanometers) on a chemically inert mica substrate via a facile chemical vapor deposition route is demonstrated. This is accomplished by an accurate control of the feeding rate of the Cr precursor and the growth temperature. Furthermore, it is revealed that the conduction behavior of the nanothick Cr2 S3 is variable with increasing thickness (from 2.6 to 4.8 nm and >7 nm) from p-type to ambipolar and then to n-type. Hereby, this work can shed light on the scalable synthesis, transport, and magnetic properties explorations of 2D magnetic materials.

3.
J Chem Phys ; 151(21): 211101, 2019 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-31822097

RESUMO

Cesium lead bromide (CsPbBr3) perovskite has attracted great attention recently for its potentials for next-generation green-color lasing devices owing to the relatively high structural stability and the high emission efficiency among the perovskite family. Herein, we explore the origins of cavity modes in CsPbBr3 microplatelets (MPs) lasers by using angle-resolved microphotoluminescence Fourier imaging technique, which is still controversial so far. In-plane Fabry-Pérot (F-P) mode lasing transition to whispering-gallery-mode (WGM) lasing is verified at room temperature, which mostly occurs in large MPs with edge length (L) over 13 µm. The F-P lasing is suppressed upon decreasing L or increasing excitation density, and the WGM lasing is predominant for all MPs at high excitation density. Furthermore, the parity and symmetry of in-plane F-P modes are classified. These results advance the fundamental understanding of lasing modes in planar microcavities as well as their applications in on-chip interconnection and quantum optics.


Assuntos
Compostos de Cálcio/química , Lasers , Óxidos/química , Titânio/química , Óptica e Fotônica , Tamanho da Partícula , Teoria Quântica , Propriedades de Superfície
4.
Adv Mater ; : e1904249, 2019 Dec 26.
Artigo em Inglês | MEDLINE | ID: mdl-31880031

RESUMO

The development of highly efficient photocatalytic systems with rapid photogenerated charge separation and high surface catalytic activity is highly desirable for the storage and conversion of solar energy, yet remains a grand challenge. Herein, a conceptionally new form of atomically dispersed Co-P3 species on CdS nanorods (CoPSA-CdS) is designed and synthesized for achieving unprecedented photocatalytic activity for the dehydrogenation of formic acid (FA) to hydrogen. X-ray absorption near edge structure, X-ray photoelectron spectroscopy, and time-resolved photoluminescence results confirm that the Co-P3 species have a unique electron-rich feature, greatly improving the efficiency of photogenerated charge separation through an interface charge effect. The in situ attenuated total reflection infrared spectra reveal that the Co-P3 species can achieve much better dissociation adsorption of FA and activation of CH bonds than traditional sulfur-coordinated Co single atom-loaded CdS nanorods (CoSSA-CdS). These two new features make CoPSA-CdS exhibit the unprecedented 50-fold higher activity in the photocatalytic dehydrogenation of FA than CoSSA-CdS, and also much better activity than the Ru-, Rh-, Pd-, or Pt-loaded CdS. Besides, CoPSA-CdS also shows the highest mass activity (34309 mmol gCo -1 h-1 ) of Co reported to date. First-principles simulation reveals that the Co-P3 species herein can form an active PHCOO intermediate for enhancing the rate-determining dissociation adsorption of FA.

5.
ACS Appl Mater Interfaces ; 11(51): 48221-48229, 2019 Dec 26.
Artigo em Inglês | MEDLINE | ID: mdl-31782301

RESUMO

Vertical heterostructures formed by stacks of two-dimensional (2D) layered materials with disparate electronic properties have attracted tremendous attention for their versatile applications. The targeted fabrication of such vertical stacks with clean interfaces and a specific stacking sequence remains challenging. Herein, we design a two-step chemical vapor deposition route for the direct synthesis of unconventional graphene/PtSe2 vertical stacks (Gr/PtSe2) on conductive Au foil substrates. Monolayer PtSe2 (1L-PtSe2) was detected to preferentially grow at the interface of the predeposited Gr layer and the Au foil substrate rather than on the Gr surface. The concurrent effect from the strong interaction of PtSe2/Au and the space confinement effect of Gr/Au are proposed to be the essential mechanisms. Particularly, this unique growth system allows us to uncover the intrinsic property of 1L-PtSe2 and the interfacial coupling effect using scanning tunneling microscopy/spectroscopy. Our work should hereby enable significant advances in the synthesis of 2D-based vertical heterostructures and in the exploration of their intrinsic interface properties.

6.
ACS Nano ; 13(9): 10085-10094, 2019 Sep 24.
Artigo em Inglês | MEDLINE | ID: mdl-31436948

RESUMO

Integrating metallic halide perovskites with established modern semiconductor technology is significant for promoting the development of application-level optoelectronic devices. To realize such devices, exploring the growth dynamics and interfacial carrier dynamics of perovskites deposited on the core materials of semiconductor technology is essential. Herein, we report the incommensurate heteroepitaxy of highly oriented single-crystal cesium lead bromide (CsPbBr3) on c-wurtzite GaN/sapphire substrates with atomically smooth surface and uniform rectangular shape by chemical vapor deposition. The CsPbBr3 microplatelet crystal exhibits green-colored lasing under room temperature and has a structural stability comparable with that grown on van der Waals mica substrates. Time-resolved photoluminescence spectroscopy studies show that the type-II CsPbBr3-GaN heterojunction effectively enhances the separation and extraction of free carriers inside CsPbBr3. These findings provide insights into the fabrication and application-level integrated optoelectronic devices of CsPbBr3 perovskites.

7.
Adv Mater ; 31(39): e1903030, 2019 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-31408551

RESUMO

2D Ruddlesden-Popper perovskites (RPPs) have aroused growing attention in light harvesting and emission applications owing to their high environmental stability. Recently, coherent light emission of RPPs was reported, however mostly from inhomologous thin films that involve cascade intercompositional energy transfer. Lasing and fundamental understanding of intrinsic laser dynamics in homologous RPPs free from intercompositional energy transfer is still inadequate. Herein, the lasing and loss mechanisms of homologous 2D (BA)2 (MA)n -1 Pbn I3 n +1 RPP thin flakes mechanically exfoliated from the bulk crystal are reported. Multicolor lasing is achieved from the large-n RPPs (n ≥ 3) in the spectral range of 620-680 nm but not from small-n RPPs (n ≤ 2) even down to 78 K. With decreasing n, the lasing threshold increases significantly and the characteristic temperature decreases as 49, 25, and 20 K for n = 5, 4, and 3, respectively. The n-engineered lasing behaviors are attributed to the stronger Auger recombination and exciton-phonon interaction as a result of the enhanced quantum confinement in the smaller-n perovskites. These results not only advance the fundamental understanding of loss mechanisms in both inhomologous and homologous RPP lasers but also provide insights into developing low-threshold, substrate-free, and multicolor 2D semiconductor microlasers.

8.
ACS Appl Mater Interfaces ; 11(22): 20566-20573, 2019 Jun 05.
Artigo em Inglês | MEDLINE | ID: mdl-31082257

RESUMO

The charge-transfer process in transition-metal dichalcogenides (TMDCs) lateral homojunction affects the electron-hole recombination process of in optoelectronic devices. However, the optical properties of the homojunction reflecting the charge-transfer process has not been observed and studied. In this work, we investigated the charge-transfer-induced emission properties based on monolayer (1L)-bilayer (2L) WSe2 lateral homojunction with dozens of nanometer monolayer region. On the one hand, the photoluminescence (PL) emission of bilayer WSe2 from the homojunction area blue shifts ∼23 and ∼31 meV for direct and indirect bandgap emission, respectively, compared with the bare WSe2 bilayer region. The blue shift of the emission spectrum in the bilayer WSe2 is ascribed to the decrease in binding energy induced by charge transfer from monolayer to bilayer. On the other hand, the energy shift shows a tendency to increase as the temperature decreases. The energy blue shift is ∼57 meV for direct bandgap emission at 80 K, which is larger than that (∼23 meV) at room temperature. The larger-energy blue shift at low temperature is derived from the larger driving force under larger band offset. Our observations of the unique optical properties induced by efficient charge transfer are very helpful for exploring novel TMDC-based optoelectronic devices.

9.
Nanoscale ; 11(7): 3145-3153, 2019 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-30724945

RESUMO

Recently, the light-matter interaction of perovskite microcavities has been widely explored for its great potential in low-threshold lasing devices. However, the mechanism of perovskite lasing remains unclear to date. In this study, we demonstrated high-quality single-mode excitonic lasing in CsPbBr3 microspheres, providing an ideal platform to study the underlying physics of lasing behavior. We show that the lasing mechanism shifts from the exciton-exciton scattering to the exciton-phonon scattering with the increase in temperature from 77 to 300 K, which was verified by temperature-dependent photoluminescence (PL), time-resolved photoluminescence (TRPL) as well as temperature-dependent Raman spectroscopy. Furthermore, by analyzing PL line width broadening with varied temperatures, we found that two different phonon modes were involved in the exciton-phonon scattering process. The scattering from the low-energy phonon (∼8.6 meV) is the dominant source of exciton-phonon coupling in the intermediate temperature range (77 to 230 K), while the high-energy phonon (∼15.3 meV) dominates from 230 K to room temperature. These results confirm the lasing mechanism in such perovskite-based micro/nano-cavities and significantly influence the development of future low-threshold lasers.

10.
Phys Rev Lett ; 121(14): 145505, 2018 Oct 05.
Artigo em Inglês | MEDLINE | ID: mdl-30339427

RESUMO

Carbon (C) doping is essential for producing semi-insulating GaN for power electronics. However, to date the nature of C doped GaN, especially the lattice site occupation, is not yet well understood. In this work, we clarify the lattice site of C in GaN using polarized Fourier-transform infrared and Raman spectroscopies, in combination with first-principles calculations. Two local vibrational modes (LVMs) at 766 and 774 cm^{-1} in C doped GaN are observed. The 766 cm^{-1} mode is assigned to the nondegenerate A_{1} mode vibrating along the c axis, whereas the 774 cm^{-1} mode is ascribed to the doubly degenerate E mode confined in the plane perpendicular to the c axis. The two LVMs are identified to originate from isolated C_{N}^{-} with local C_{3v} symmetry. Experimental data and calculations are in outstanding agreement both for the positions and the intensity ratios of the LVMs. We thus provide unambiguous evidence of the substitutional C atoms occupying the N site with a -1 charge state in GaN and therefore bring essential information to a long-standing controversy.

11.
Small ; 14(35): e1801938, 2018 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-30066432

RESUMO

Lead Iodide (PbI2 ) is a layered semiconductor with direct band gap holding great promises in green light emission and detection devices. Recently, PbI2 planar lasers are demonstrated using hexagonal whispering-gallery-mode microcavities, but the lasing threshold is quite high. In this work, lasing from vapor phase deposition derived PbI2 trapezoidal nanoplatelets (NPs) with threshold that is at least an order of magnitude lower than the previous value is reported. The growth mechanism of the trapezoidal NPs is explored and attributed to the synergistic effects of van der Waals interactions and lattice mismatching. The lasing is enabled by the population inversion of n = 1 excitons and the optical feedback is provided by the Fabry-Pérot oscillation between the side facets of trapezoidal NPs. The findings not only advance the understanding of growth and photophysics mechanism of PbI2 nanostructures but also provide ideas to develop low threshold ultrathin lasers.

12.
ACS Nano ; 12(9): 9390-9396, 2018 Sep 25.
Artigo em Inglês | MEDLINE | ID: mdl-30133255

RESUMO

The realization of low-energy-consumption lasers based on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) is crucial for the development of optical communications, flexible displays, and lasers on the chip level. However, among the as-demonstrated TMDC-based lasers so far, the gain materials are mainly achieved by a mechanical exfoliation approach accompanied by poor reproducibility and controllability. In this work, we report a controllable design for generating large-scale lasing from chemical vapor deposition (CVD)-derived high-quality monolayer MoS2 film. Strong continuous-wave optically driven whispering-gallery-mode lasing is achieved in a wide temperature range from 77 to 400 K. The eminent lasing performances result from the strong spatial confinement of carriers and the enhanced efficiency of spontaneous emission owing to the lensing and screening effects of silica microsphere cavities. These findings not only advance the fundamental understanding of 2D lasing effects but also provide solutions to fabricate low-cost, scalable, and integratable TMDC-based lasers.

13.
Adv Mater ; 30(31): e1801805, 2018 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-29923237

RESUMO

1D nanowires of all-inorganic lead halide perovskites represent a good architecture for the development of polarization-sensitive optoelectronic devices due to their high absorption efficient, emission yield, and dielectric constants. However, among as-fabricated perovskite nanowires with the lateral dimensions of hundreds nanometers so far, the optical anisotropy is hindered and rarely explored owing to the invalidating of electrostatic dielectric mismatch in the physical dimensions. Here, well-aligned CsPbBr3 and CsPbCl3 nanowires with thickness T down to 15 and 7 nm, respectively, are synthesized using a vapor phase van der Waals epitaxial method. Strong emission anisotropy with polarization ratio up to ≈0.78 is demonstrated in the nanowires with T < 40 nm due to the electrostatic dielectric confinement. With the increasing of thickness, the polarization ratio remarkably reduces monotonously to ≈0.17 until T ≈140 nm; and further oscillates in a small amplitude owing to the wave characteristic of light. These findings not only represent a demonstration of perovskite-based polarization-sensitive light sources, but also advance fundamental understanding of their polarization properties of perovskite nanowires.

14.
Nano Lett ; 18(6): 3335-3343, 2018 06 13.
Artigo em Inglês | MEDLINE | ID: mdl-29722986

RESUMO

Manipulating strong light-matter interaction in semiconductor microcavities is crucial for developing high-performance exciton polariton devices with great potential in next-generation all-solid state quantum technologies. In this work, we report surface plasmon enhanced strong exciton-photon interaction in CH3NH3PbBr3 perovskite nanowires. Characteristic anticrossing behaviors, indicating a Rabi splitting energy up to ∼564 meV, are observed near exciton resonance in hybrid perovskite nanowire/SiO2/Ag cavity at room temperature. The exciton-photon coupling strength is enhanced by ∼35% on average, which is mainly attributed to surface plasmon induced localized excitation field redistribution. Further, systematic studies on SiO2 thickness and nanowire dimension dependence of exciton-photon interaction are presented. These results provide new avenues to achieve extremely high coupling strengths and push forward the development of electrically pumped and ultralow threshold small lasers.

15.
J Phys Chem Lett ; 9(7): 1655-1662, 2018 Apr 05.
Artigo em Inglês | MEDLINE | ID: mdl-29533623

RESUMO

Mixed-dimensional van der Waals (vdW) heterostructures between one-dimensional (1D) perovskite nanowires and two-dimensional (2D) transition metal dichalcogenides (TMDCs) hold great potential for novel optoelectronics and light-harvesting applications. However, the ultrafast carrier dynamics between the 1D perovskite nanowires and 2D TMDCs are currently not well understood, which is critical for related optoelectronic applications. Here we demonstrate vdW heterostructures of CsPbBr3 nanowire/monolayer MoS2 and CsPbBr3 nanowire/monolayer WSe2 and further present systematic investigations on their charge transfer dynamics. We show that CsPbBr3/MoS2 and CsPbBr3/WSe2 are type-I and type-II heterostructures, respectively. Both electrons and holes transfer from CsPbBr3 to MoS2 with an efficiency of 71%. As a contrast, holes transfer from CsPbBr3 to WSe2 with a carrier transfer efficiency of 70% and electrons transfer inversely within 7 ps. The ultrafast and efficient charge transfer in the 1D/2D perovskite-TMDC heterostructures suggest great promise in light emission, photodetector, and photovoltaic devices.

16.
J Phys Chem Lett ; 9(5): 1124-1132, 2018 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-29432017

RESUMO

The construction of hybrid perovskites (both two-dimensional (2D) and three-dimensional (3D)) has attracted intensive research interest recently. Here, a facile, two-step consecutive deposition method was developed for the first time to grow a hierarchical quasi-2D/3D perovskite superstructure, with an oriented quasi-2D ((BA)2(MA)n-1PbnI3n+1) perovskite nanosheet (NS) perpendicularly aligned on 3D perovskites. The superstructures are found to be mixtures of multiple perovskite phases, with n = 2, 3, 4 and 3D perovskite; however, the n value was naturally increased from top to the bottom, which is distinct from many other works. We found that the concentration gradient, namely, the initial ratio and amount of BAI/MAI, collectively contributes to the spatially confined nucleation and growth of oriented quasi-2D superstructure perovskite on 3D perovskites. An efficient charge carrier transfer was demonstrated from small-n to large-n phases in this perovskite superstructure, indicating a different type of energy funnel from top to bottom.

17.
Small ; 14(9)2018 03.
Artigo em Inglês | MEDLINE | ID: mdl-29320610

RESUMO

Recently, organometal halide perovskite-based optoelectronics, particularly lasers, have attracted intensive attentions because of its outstanding spectral coherence, low threshold, and wideband tunability. In this work, high-quality CH3 NH3 PbBr3 single crystals with a unique shape of cube-corner pyramids are synthesized on mica substrates using chemical vapor deposition method. These micropyramids naturally form cube-corner cavities, which are eminent candidates for small-sized resonators and retroreflectors. The as-grown perovskites show strong emission ≈530 nm in the vertical direction at room temperature. A special Fabry-Pérot (F-P) mode is employed to interpret the light confinement in the cavity. Lasing from the perovskite pyramids is observed from 80 to 200 K, with threshold ranging from ≈92 µJ cm-2 to 2.2 mJ cm-2 , yielding a characteristic temperature of T0 = 35 K. By coating a thin layer of Ag film, the threshold is reduced from ≈92 to 26 µJ cm-2 , which is accompanied by room temperature lasing with a threshold of ≈75 µJ cm-2 . This work advocates the prospect of shape-engineered perovskite crystals toward developing micro-sized optoelectronic devices and potentially investigating light-matter coupling in quantum optics.

18.
ACS Appl Mater Interfaces ; 9(33): 27402-27408, 2017 Aug 23.
Artigo em Inglês | MEDLINE | ID: mdl-28796477

RESUMO

MoS2 as atomically thin semiconductor is highly sensitive to ambient atmosphere (e.g., oxygen, moisture, etc.) in optical and electrical properties. Here we report a controlled gas molecules doping of monolayer MoS2 via atomic-layer-deposited Al2O3 films. The deposited Al2O3 films, in the shape of nanospheres, can effectively control the contact areas between ambient atmosphere and MoS2 that allows precise modulation of gas molecules doping. By analyzing photoluminescence (PL) emission spectra of MoS2 with different thickness of Al2O3, the doped carrier concentration is estimated at ∼2.7 × 1013 cm-2 based on the mass action model. Moreover, time-dependent PL measurements indicate an incremental stability of single layer MoS2 as the thicknesses of Al2O3 capping layer increase. Effective control of gas molecules doping in monolayer MoS2 provides us a valuable insight into the applications of MoS2 based optical and electrical devices.

19.
J Phys Chem Lett ; 8(18): 4431-4438, 2017 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-28845670

RESUMO

Quasi-two-dimensional Ruddlesden-Popper perovskites driving carrier self-separation have rapidly advanced the development of high-performance optoelectronic devices. However, insightful understanding of carrier dynamics in the perovskites is still inadequate. The distribution of multiple perovskite phases, crucial for carrier separation, is controversial. Here we report a systematic study on carrier dynamics of spin-coated (C6H5CH2CH2NH3)2(CH3NH3)n-1PbnI3n+1 (n = 3 and 5) perovskite thin films. Efficient electrons transfer from small-n to large-n perovskite phases, and holes transfer reversely with time scales from ∼0.3 to 30.0 ps. The multiple perovskite phases are arranged perpendicularly to substrate from small to large n and also coexist randomly in the same horizontal planes. Further, the carrier separation dynamics is tailored by engineering the crystalline structure of the perovskite film, which leads to controllable emission properties. These results have important significance for the design of optoelectronic devices from solar cells, light-emitting diodes, lasers, and so forth.

20.
Nanoscale ; 9(24): 8281-8287, 2017 Jun 22.
Artigo em Inglês | MEDLINE | ID: mdl-28585960

RESUMO

Recently, organic-inorganic lead halide perovskites have gained great attention for their breakthrough in photovoltaic and optoelectronics. However, their thermal transport properties that affect the device lifetime and stability are still rarely explored. In this work, the thermal conductivity properties of single crystal CH3NH3PbI3 platelets grown by chemical vapor deposition are studied via non-contact micro-photoluminescence (PL) spectroscopy. We developed a measurement methodology and derived expressions suitable for the thermal conductivity extraction for micro-sized perovskites. The room temperature thermal conductivity of ∼0.14 ± 0.02 W m-1 K-1 is extracted from the dependence of the PL peak energy on the excitation laser power. On changing the film thickness from 80 to 400 nm, the thermal conductivity does not show noticeable variations, indicating the minimal substrate effects due to the advantage of the suspended configuration. The ultra-low thermal conductivity of perovskites, especially thin films, suggests their promising applications for thermal isolation, such as thermal insulation and thermo-electricity.

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