More Like this

1. Fluorescence Microscopy of Single Lead Bromide Nanocrystals Reveals Sharp Transitions during Their Transformation to Methylammonium Lead Bromide

   https://doi.org/10.1039/C8TC06470A  

   Yin, Bo ; Cavin, John ; Wang, Dong ; Khan, Daniel ; Shen, Meikun ; Laing, Craig ; Mishra, Rohan ; Sadtler, Bryce ( January 2019 , Journal of Materials Chemistry C)

   Control over the nucleation and growth of lead-halide perovskite crystals is critical to obtain semiconductor films with high quantum yields in optoelectronic devices. In this report, we use the change in fluorescence brightness to image the transformation of individual lead bromide (PbBr 2 ) nanocrystals to methylammonium lead bromide (CH 3 NH 3 PbBr 3 ) via intercalation of CH 3 NH 3 Br. Analyzing this reaction one nanocrystal at a time reveals information that is masked when the fluorescence intensity is averaged over many particles. Sharp rises in the intensity of single nanocrystals indicate they transform much faster than the time it takes for the ensemble average to transform. While the ensemble reaction rate increases with increasing CH 3 NH 3 Br concentration, the intensity rises for individual nanocrystals are insensitive to the CH 3 NH 3 Br concentration. To explain these observations, we propose a phase-transformation model in which the reconstructive transitions necessary to convert a PbBr 2 nanocrystal into CH 3 NH 3 PbBr 3 initially create a high energy barrier for ion intercalation. A critical point in the transformation occurs when the crystal adopts the perovskite phase, at which point the activation energy for further ionmore »intercalation becomes progressively smaller. Monte Carlo simulations that incorporate this change in activation barrier into the likelihood of reaction events reproduce key experimental observations for the intensity trajectories of individual particles. The insights gained from this study may be used to further control the crystallization of CH 3 NH 3 PbBr 3 and other solution-processed semiconductors.« less
2. Doping and ion substitution in colloidal metal halide perovskite nanocrystals

   https://doi.org/10.1039/C9CS00790C  

   Lu, Cheng-Hsin ; Biesold-McGee, Gill V. ; Liu, Yijiang ; Kang, Zhitao ; Lin, Zhiqun ( July 2020 , Chemical Society Reviews)

   The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX 3 , where A is a monovalent cation (which can be either organic ( e.g. , CH 3 NH 3 + (MA), CH(NH 2 ) 2 + (FA)) or inorganic ( e.g. , Cs + )), B is a divalent metal cation (usually Pb 2+ ), and X is a halogen anion (Cl − , Br − , I − ). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties ( e.g. , absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A′, B′, or X′ site ions into themore »A, B, or X sites of ABX 3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed.« less
3. Peculiarities of crystal structures and photophysical properties of Ga ^III /Ln ^III metallacrowns with a non-planar [12-MC-4] core

   https://doi.org/10.1039/c9qi01647c  

   Nguyen, Tu N. ; Eliseeva, Svetlana V. ; Chow, Chun Y. ; Kampf, Jeff W. ; Petoud, Stéphane ; Pecoraro, Vincent L. ( March 2020 , Inorganic Chemistry Frontiers)

   A new series of gallium( iii )/lanthanide( iii ) metallacrown (MC) complexes ( Ln-1 ) was synthesized by the direct reaction of salicylhydroxamic acid (H 3 shi) with Ga III and Ln III nitrates in a CH 3 OH/pyridine mixture. X-ray single crystal analysis revealed two types of structures depending on whether the nitrate counterion coordinate or not to the Ln III : [LnGa 4 (shi) 4 (H 2 shi) 2 (py) 4 (NO 3 )](py) 2 (Ln = Gd III , Tb III , Dy III , Ho III ) and [LnGa 4 (shi) 4 (H 2 shi) 2 (py) 5 ](NO 3 )(py) (Ln = Er III , Tm III , Yb III ). The representative Tb-1 and Yb-1 MCs consist of a Tb/YbGa 4 core with four [Ga III –N–O] repeating units forming a non-planar ring that coordinates the central Ln III through the oxygen atoms of the four shi 3− groups. Two H 2 shi − groups bridge the Ln III to the Ga III ring ions. The Yb III in Yb-1 is eight-coordinated while the ligation of the nine-coordinated Tb III in Tb-1 is completed by one chelating nitrate ion. Ln-1 complexes in the solidmore »state showed characteristic sharp f–f transitions in the visible (Tb, Dy) and near-infrared (Dy, Ho, Er, Yb) spectral ranges upon excitation into the ligand-centered electronic levels at 350 nm. Observed luminescence lifetimes and absolute quantum yields were collected and discussed. For Yb-1 , luminescence data were also acquired in CH 3 OH and CD 3 OD solutions and a more extensive analysis of photophysical properties was performed. This work demonstrates that while obtaining highly luminescent lanthanide( iii ) MCs via a direct synthesis is feasible, many factors such as molar absorptivities, triplet state energies, non-radiative deactivations through vibronic coupling with overtones of O–H, N–H, and C–H oscillators and crystal packing will strongly contribute to the luminescent properties and should be carefully considered.« less
4. RTD Light Emission around 1550 nm with IQE up to 6% at 300 K

   https://doi.org/10.1109/DRC50226.2020.9135175  

   Brown, E. R. ; Zhang, W-D. ; Fakhimi, P. ; Growden, T. A. ; Berger, P.R. ( June 2020 , IEEE Device Research Conference (DRC), Columbus, OH (June 22-24, 2020))

   Resonant tunneling diodes (RTDs) have come full-circle in the past 10 years after their demonstration in the early 1990s as the fastest room-temperature semiconductor oscillator, displaying experimental results up to 712 GHz and fmax values exceeding 1.0 THz [1]. Now the RTD is once again the preeminent electronic oscillator above 1.0 THz and is being implemented as a coherent source [2] and a self-oscillating mixer [3], amongst other applications. This paper concerns RTD electroluminescence – an effect that has been studied very little in the past 30+ years of RTD development, and not at room temperature. We present experiments and modeling of an n-type In0.53Ga0.47As/AlAs double-barrier RTD operating as a cross-gap light emitter at ~300K. The MBE-growth stack is shown in Fig. 1(a). A 15-μm-diam-mesa device was defined by standard planar processing including a top annular ohmic contact with a 5-μm-diam pinhole in the center to couple out enough of the internal emission for accurate free-space power measurements [4]. The emission spectra have the behavior displayed in Fig. 1(b), parameterized by bias voltage (VB). The long wavelength emission edge is at  = 1684 nm - close to the In0.53Ga0.47As bandgap energy of Ug ≈ 0.75 eV at 300 K.more »The spectral peaks for VB = 2.8 and 3.0 V both occur around  = 1550 nm (h = 0.75 eV), so blue-shifted relative to the peak of the “ideal”, bulk InGaAs emission spectrum shown in Fig. 1(b) [5]. These results are consistent with the model displayed in Fig. 1(c), whereby the broad emission peak is attributed to the radiative recombination between electrons accumulated on the emitter side, and holes generated on the emitter side by interband tunneling with current density Jinter. The blue-shifted main peak is attributed to the quantum-size effect on the emitter side, which creates a radiative recombination rate RN,2 comparable to the band-edge cross-gap rate RN,1. Further support for this model is provided by the shorter wavelength and weaker emission peak shown in Fig. 1(b) around = 1148 nm. Our quantum mechanical calculations attribute this to radiative recombination RR,3 in the RTD quantum well between the electron ground-state level E1,e, and the hole level E1,h. To further test the model and estimate quantum efficiencies, we conducted optical power measurements using a large-area Ge photodiode located ≈3 mm away from the RTD pinhole, and having spectral response between 800 and 1800 nm with a peak responsivity of ≈0.85 A/W at  =1550 nm. Simultaneous I-V and L-V plots were obtained and are plotted in Fig. 2(a) with positive bias on the top contact (emitter on the bottom). The I-V curve displays a pronounced NDR region having a current peak-to-valley current ratio of 10.7 (typical for In0.53Ga0.47As RTDs). The external quantum efficiency (EQE) was calculated from EQE = e∙IP/(∙IE∙h) where IP is the photodiode dc current and IE the RTD current. The plot of EQE is shown in Fig. 2(b) where we see a very rapid rise with VB, but a maximum value (at VB= 3.0 V) of only ≈2×10-5. To extract the internal quantum efficiency (IQE), we use the expression EQE= c ∙i ∙r ≡ c∙IQE where ci, and r are the optical-coupling, electrical-injection, and radiative recombination efficiencies, respectively [6]. Our separate optical calculations yield c≈3.4×10-4 (limited primarily by the small pinhole) from which we obtain the curve of IQE plotted in Fig. 2(b) (right-hand scale). The maximum value of IQE (again at VB = 3.0 V) is 6.0%. From the implicit definition of IQE in terms of i and r given above, and the fact that the recombination efficiency in In0.53Ga0.47As is likely limited by Auger scattering, this result for IQE suggests that i might be significantly high. To estimate i, we have used the experimental total current of Fig. 2(a), the Kane two-band model of interband tunneling [7] computed in conjunction with a solution to Poisson’s equation across the entire structure, and a rate-equation model of Auger recombination on the emitter side [6] assuming a free-electron density of 2×1018 cm3. We focus on the high-bias regime above VB = 2.5 V of Fig. 2(a) where most of the interband tunneling should occur in the depletion region on the collector side [Jinter,2 in Fig. 1(c)]. And because of the high-quality of the InGaAs/AlAs heterostructure (very few traps or deep levels), most of the holes should reach the emitter side by some combination of drift, diffusion, and tunneling through the valence-band double barriers (Type-I offset) between InGaAs and AlAs. The computed interband current density Jinter is shown in Fig. 3(a) along with the total current density Jtot. At the maximum Jinter (at VB=3.0 V) of 7.4×102 A/cm2, we get i = Jinter/Jtot = 0.18, which is surprisingly high considering there is no p-type doping in the device. When combined with the Auger-limited r of 0.41 and c ≈ 3.4×10-4, we find a model value of IQE = 7.4% in good agreement with experiment. This leads to the model values for EQE plotted in Fig. 2(b) - also in good agreement with experiment. Finally, we address the high Jinter and consider a possible universal nature of the light-emission mechanism. Fig. 3(b) shows the tunneling probability T according to the Kane two-band model in the three materials, In0.53Ga0.47As, GaAs, and GaN, following our observation of a similar electroluminescence mechanism in GaN/AlN RTDs (due to strong polarization field of wurtzite structures) [8]. The expression is Tinter = (2/9)∙exp[(-2 ∙Ug 2 ∙me)/(2h∙P∙E)], where Ug is the bandgap energy, P is the valence-to-conduction-band momentum matrix element, and E is the electric field. Values for the highest calculated internal E fields for the InGaAs and GaN are also shown, indicating that Tinter in those structures approaches values of ~10-5. As shown, a GaAs RTD would require an internal field of ~6×105 V/cm, which is rarely realized in standard GaAs RTDs, perhaps explaining why there have been few if any reports of room-temperature electroluminescence in the GaAs devices. [1] E.R. Brown,et al., Appl. Phys. Lett., vol. 58, 2291, 1991. [5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981). [2] M. Feiginov et al., Appl. Phys. Lett., 99, 233506, 2011. [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995). [3] Y. Nishida et al., Nature Sci. Reports, 9, 18125, 2019. [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961). [4] P. Fakhimi, et al., 2019 DRC Conference Digest. [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018). [5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981). [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995). [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961). [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018).« less
5. Synthesis and molecular structure of model silica-supported tungsten oxide catalysts for oxidative coupling of methane (OCM)

   https://doi.org/10.1039/D0CY00289E  

   Kiani, Daniyal ; Sourav, Sagar ; Wachs, Israel E. ; Baltrusaitis, Jonas ( May 2020 , Catalysis Science & Technology)

   The molecular and electronic structures and chemical properties of the active sites on the surface of supported Na 2 WO 4 /SiO 2 catalysts used for oxidative coupling of methane (OCM) are poorly understood. Model SiO 2 -supported, Na-promoted tungsten oxide catalysts (Na–WO x /SiO 2 ) were systematically prepared using various Na- and W-precursors using carefully controlled Na/W molar ratios and examined with in situ Raman, UV-vis DR, CO 2 -TPD-DRIFT and NH 3 -TPD-DRIFT spectroscopies. The traditionally-prepared catalysts corresponding to 5% Na 2 WO 4 nominal loading, with Na/W molar ratio of 2, were synthesized from the aqueous Na 2 WO 4 ·2H 2 O precursor. After calcination at 800 °C, the initially amorphous SiO 2 support crystallized to the cristobalite phase and the supported sodium tungstate phase consisted of both crystalline Na 2 WO 4 nanoparticles (Na/W = 2) and dispersed phase Na–WO 4 surface sites (Na/W < 2). On the other hand, the catalysts prepared via a modified impregnation method using individual precursors of NaOH + AMT, such that the Na/W molar ratio remained well below 2, resulted in: (i) SiO 2 remaining amorphous (ii) only dispersed phase Na–WO 4 surface sites. The dispersed Na–WO 4more »surface sites were isolated, more geometrically distorted, less basic in nature, and more reducible than the crystalline Na 2 WO 4 nanoparticles. The CH 4 + O 2 -TPSR results reveal that the isolated, dispersed phase Na–WO 4 surface sites were significantly more C 2 selective, but slightly less active than the traditionally-prepared catalysts that contain crystalline Na 2 WO 4 nanoparticles (Na/W = 2). These findings demonstrate that the isolated, dispersed phase Na–WO 4 sites on the SiO 2 support surface are the selective-active sites for the OCM reaction.« less