Orientation-Controlled Construction of Superstructures of Atomically-Flat Nanocrystals: Pushing the Limits of Ultra-Thin Colloidal Gain Media

We propose and demonstrate a method for the construction of highly uniform, multilayered, orientation-controlled superstructures of CdSe/CdZnS core/shell colloidal nanoplatelets (NPLs) using bi-phase liquid interface. These atomically-flat nanocrystals are sequentially deposited, all face-down onto a solid substrate, into slabs having monolayer-precise thickness and excellent homogeneity over several tens of cm2 areas. Owing to the near-unity surface coverage and film uniformity of this deposition technique, amplified spontaneous emission (ASE) is observed from an uncharacteristically thin colloidal film having only 6 layers of NPLs, which corresponds to a mere 42 nm thickness. Furthermore, systematic studies of optical gain properties of these NPL superstructures constructed having precise numbers of NPL layers tuned from 6 to 15 revealed the reduction in the gain threshold with the increasing number of NPL monolayers, along with a continuous spectral shift in the position of the ASE peak (by ~18 nm). These observations can be well explained by the variation of the optical field confinement factor with the NPL waveguide thickness and propagation wavelength. This work demonstrates the possibility of fabricating thickness-tunable, large-area three-dimensional superstructures made of NPL building blocks, which can be additively constructed one monolayer at a time. The proposed technique can also be extended to build hybrid NPL films of mixed orientations and allow for precise large-area device engineering.Comment: 23 pages, 4 figure

Self-Resonant u-Lasers of Colloidal Quantum Wells Constructed by Direct Deep Patterning

Here, the first account of self-resonant fully-colloidal u-lasers made from colloidal quantum well (CQW) solution is reported. A deep patterning technique is developed to fabricate well-defined high aspect-ratio on-chip CQW resonators made of grating waveguides and in-plane reflectors. CQWs of the patterned layers are closed-packed with sharp edges and residual-free lifted-off surfaces. Additionally, the method is successfully applied to various nanoparticles including colloidal quantum dots and metal nanoparticles. It is observed that the patterning process does not affect the nanocrystals (NCs) immobilized in the attained patterns and different physical and chemical properties of the NCs remain pristine. Thanks to capability of the proposed patterning method, patterns of NCs with sub-wavelength lateral feature size and micron-scale height are fabricated in the aspect ratios of 1:15 (<100 nm lateral patterned features to >1.5 {\mu}m film thickness). The fabricated waveguide-coupled laser, enabling tight optical confinement, assures in-plane lasing. The spectral characteristics of the designed CQW resonator structure are well supported with a numerical model of full electromagnetic solutions. Such directly deep-patterned self-resonant u-lasers of CQWs hold great promise for on-chip integration to photonic circuits

Single-mode lasing from a single 7 nm thick monolayer of colloidal quantum wells in a monolithic microcavity

In this work, we report the first account of monolithically-fabricated vertical cavity surface emitting lasers (VCSELs) of densely-packed, orientation-controlled, atomically flat colloidal quantum wells (CQWs) using a self-assembly method and demonstrate single-mode lasing from a record thin colloidal gain medium with a film thickness of 7 nm under femtosecond optical excitation. We used specially engineered CQWs to demonstrate these hybrid CQW-VCSELs consisting of only a few layers to a single monolayer of CQWs and achieved the lasing from these thin gain media by thoroughly modeling and implementing a vertical cavity consisting of distributed Bragg reflectors with an additional dielectric layer for mode tuning. Accurate spectral and spatial alignment of the cavity mode with the CQW films was secured with the help of full electromagnetic computations. While overcoming the long-pending problem of limited electrical conductivity in thicker colloidal films, such ultra-thin colloidal gain media can help enabling fully electrically-driven colloidal lasers

Highly Stable, Near-Unity Efficiency Atomically Flat Semiconductor Nanocrystals of CdSe/ZnS Hetero-Nanoplatelets Enabled by ZnS-Shell Hot-Injection Growth

Colloidal semiconductor nanoplatelets (NPLs) offer important benefits in nanocrystal optoelectronics with their unique excitonic properties. For NPLs, colloidal atomic layer deposition (c-ALD) provides the ability to produce their core/shell heterostructures. However, as c-ALD takes place at room temperature, this technique allows for only limited stability and low quantum yield. Here, highly stable, near-unity efficiency CdSe/ZnS NPLs are shown using hot-injection (HI) shell growth performed at 573 K, enabling routinely reproducible quantum yields up to 98%. These CdSe/ZnS HI-shell hetero-NPLs fully recover their initial photoluminescence (PL) intensity in solution after a heating cycle from 300 to 525 K under inert gas atmosphere, and their solid films exhibit 100% recovery of their initial PL intensity after a heating cycle up to 400 K under ambient atmosphere, by far outperforming the control group of c-ALD shell-coated CdSe/ZnS NPLs, which can sustain only 20% of their PL. In optical gain measurements, these core/HI-shell NPLs exhibit ultralow gain thresholds reaching approximate to 7 mu J cm(-2). Despite being annealed at 500 K, these ZnS-HI-shell NPLs possess low gain thresholds as small as 25 mu J cm(-2). These findings indicate that the proposed 573 K HI-shell-grown CdSe/ZnS NPLs hold great promise for extraordinarily high performance in nanocrystal optoelectronics

Determination of thermodynamic properties of aluminum based binary and ternary alloys

In the present work, the Gibbs-Thomson coefficient, solid-liquid and solid-solid interfacial energies and grain boundary energy of a solid Al solution in the Al-Cu-Si eutectic system were determined from the observed grain boundary groove shapes by measuring the thermal conductivity of the solid and liquid phases and temperature gradient. Some thermodynamic properties such as the enthalpy of fusion, entropy of fusion, the change of specific heat from liquid to solid and the electrical conductivity of solid phases at their melting temperature were also evaluated by using the measured values of relevant data for Al-Cu, Al-Si, Al-Mg, Al-Ni, Al-Ti, Al-Cu-Ag, Al-Cu-Si binary and ternary alloys. (C) 2015 Elsevier B.V. All rights reserved

The experimental determination of thermophysical properties of intermetallic CuAl2 phase in equilibrium with (Al plus Cu plus Si) liquid

The equilibrated grain boundary groove shapes of solid CuAl2 in equilibrium with (Al + Cu + Si) eutectic liquid were observed from a quenched sample by using a radial heat flow apparatus. The Gibbs-Thomson coefficient, (solid + liquid) interfacial energy and grain boundary energy of the solid CuAl2 were determined from these observed shapes. The thermal conductivity of the eutectic solid and the thermal conductivity ratio of eutectic liquid to the eutectic solid in the (Al + 26.82 wt.% Cu + 5.27 wt.% Si) eutectic alloy at its eutectic melting temperature were also measured with a radial heat flow apparatus and a Bridgman-type growth apparatus, respectively. The three phases of (Al + Cu + Si) alloy have detected as Al solution, Si and theta (CuAl2) phases with EDX composition analysis and the microstructure of these phases were photographed by SEM. (C) 2016 Elsevier Ltd. All rights reserved

Experimental determination of interfacial energies for solid Sn in equilibrium with Sn-Mg-Zn liquid

The equilibrated grain boundary groove shapes of solid Sn in equilibrium with Sn-Mg-Zn liquid were observed from a quenched sample by using a radial heat flow apparatus. The Gibbs-Thomson coefficient, solid-liquid interfacial energy and grain boundary energy of solid Sn were determined from the observed grain boundary groove shapes. The thermal conductivity of the eutectic solid phase for Sn-8.12 at% Mg-4.97 at% Zn alloy and the thermal conductivity ratio of the liquid phase to the solid phase for Sn-8.12 at% Mg-4.97 at% Zn alloy at eutectic temperature were also measured with a radial heat flow apparatus and a Bridgman-type growth apparatus, respectively. The Gibbs-Thomson coefficient, solid-liquid interfacial energy and grain boundary energy of solid Sn in equilibrium with Sn-Mg-Zn liquid were determined to be (8.3 +/- 0.6)x10(-8) Km, (118.5 +/- 14.2)x10(-3) Jm(-2) and (225.1 +/- 29.3)x10(-3) J m(-2) respectively from observed grain boundary groove shapes. A comparison of present results for solid Sn in the Sn-8.12 at% Mg-4.97 at% Zn alloy with the results obtained in previous works for similar solid Sn in equilibrium with different binary or ternary liquid was made

Thermal conductivity and interfacial energy of solid Bi in the Bi-Ag eutectic system

The equilibrated grain boundary groove shapes for solid Bi (Bi-2.87 at.%Ag) in equilibrium with Bi-Ag eutectic liquid have been observed from quenched sample with a radial heat flow apparatus. The Gibbs-Thomson coefficient, solid-liquid interfacial energy and grain boundary energy of solid Bi have been determined from the observed grain boundary groove shapes. The variation of thermal conductivity with temperature for eutectic solid phase (Bi-4.7 at.%Ag) has been measured. The ratio of thermal conductivity of equilibrated eutectic liquid phase to eutectic solid phase has also been measured with a Bridgman-type growth apparatus at the melting temperature. The Gibbs-Thomson coefficient, solid-liquid interfacial energy and grain boundary energy of solid Bi in equilibrium with Bi-Ag eutectic liquid were determined to be (9.2 +/- A 0.6) x 10(-8) K m, (52.7 +/- A 6.3) x 10(-3) J m(-2) and (102.4 +/- A 13.3) x 10(-3) J m(-2), respectively, from observed grain boundary groove shapes