@article{Zhao2021,

title = {Enhanced Carrier Transport in Strongly Coupled, Epitaxially Fused CdSe Nanocrystal Solids},

author = {Qinghua Zhao and Guillaume Gouget and Jiacen Guo and Shengsong Yang and Tianshuo Zhao and Daniel B. Straus and Chengyang Qian and Nuri Oh and Han Wang and Christopher B. Murray and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.1c00860},

doi = {10.1021/acs.nanolett.1c00860},

year  = {2021},

date = {2021-04-01},

journal = {Nano Letters},

volume = {21},

number = {7},

pages = {3318–3324},

abstract = {Strongly coupled, epitaxially fused colloidal nanocrystal (NC) solids are promising solution-processable semiconductors to realize optoelectronic devices with high carrier mobilities. Here, we demonstrate sequential, solid-state cation exchange reactions to transform epitaxially connected PbSe NC thin films into Cu2Se nanostructured thin-film intermediates and then successfully to achieve zinc-blende, CdSe NC solids with wide epitaxial necking along {100} facets. Transient photoconductivity measurements probe carrier transport at nanometer length scales and show a photoconductance of 0.28(1) cm2 V–1 s–1, the highest among CdSe NC solids reported. Atomic-layer deposition of a thin Al2O3 layer infiltrates and protects the structure from fusing into a polycrystalline thin film during annealing and further improves the photoconductance to 1.71(5) cm2 V–1 s–1 and the diffusion length to 760 nm. We fabricate field-effect transistors to study carrier transport at micron length scales and realize high electron mobilities of 35(3) cm2 V–1 s–1 with on–off ratios of 106 after doping.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kagan2020b,

title = {Self-assembly for electronics},

author = {Cherie R. Kagan and Taeghwan Hyeon and Dae-Hyeong Kim and Ricardo Ruiz and Maryann C. Tung and H.-S. Philip Wong },

url = {https://link.springer.com/article/10.1557/mrs.2020.248},

doi = {10.1557/mrs.2020.248},

year  = {2020},

date = {2020-12-24},

journal = {MRS Bulletin},

volume = {45},

pages = {807–814},

abstract = {Self-assembly, a process in which molecules, polymers, and particles are driven by local interactions to organize into patterns and functional structures, is being exploited in advancing silicon electronics and in emerging, unconventional electronics. Silicon electronics has relied on lithographic patterning of polymer resists at progressively smaller lengths to scale down device dimensions. Yet, this has become increasingly difficult and costly. Assembly of block copolymers and colloidal nanoparticles allows resolution enhancement and the definition of essential shapes to pattern circuits and memory devices. As we look to a future in which electronics are integrated at large numbers and in new forms for the Internet of Things and wearable and implantable technologies, we also explore a broader material set. Semiconductor nanoparticles and biomolecules are prized for their size-, shape-, and composition-dependent properties and for their solution-based assembly and integration into devices that are enabling unconventional manufacturing and new device functions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Guo2019,

title = {Chemo- and Thermomechanically Configurable 3D Optical Metamaterials Constructed from Colloidal Nanocrystal Assemblies},

author = {Jiacen Guo and Ji-Young Kim and Mingliang Zhang and Haonan Wang and Aaron Stein and Christopher B. Murray and Nicholas A. Kotov and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/10.1021/acsnano.9b08452},

doi = {10.1021/acsnano.9b08452},

year  = {2019},

date = {2019-12-26},

journal = {ACS Nano},

volume = {14},

number = {2},

pages = {1427-1435},

abstract = {Nanofabrication has limited most optical metamaterials to 2D or, often with multiple patterning steps, simple 3D meta-atoms that typically have limited built-in tunability. Here, with a one-step scalable patterning process, we exploit the chemical addressability and structural adaptability of colloidal Au nanocrystal assemblies to transform 2D nanocrystal/Ti bilayers into complex, 3D-structured meta-atoms and to thermally direct their shape morphing and alter their optical properties. By tailoring the length, number, and curvature of 3D helical structures in each meta-atom, we create large-area metamaterials with chiroptical responses of as high as ∼40% transmission difference between left-hand (LCP) and right-hand (RCP) circularly polarized light (ΔT = TRCP – TLCP) that are suitable for broadband circular polarizers and, upon thermally configuring their shape, switch the polarity of polarization rotation. These 3D optical metamaterials provide prototypes for low-cost, large-scale fabrication of optical metamaterials for ultrathin lenses, polarizers, and waveplates.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Chen2019,

title = {Designing Strong Optical Absorbers via Continuous Tuning of Interparticle Interaction in Colloidal Gold Nanocrystal Assemblies},

author = {Wenxiang Chen and Jiacen Guo and Qinghua Zhao and Prashanth Gopalan and Aaron T. Fafarman and Austin Keller and Mingliang Zhang and Yaoting Wu and Christopher B. Murray and Cherie R. Kagan},

doi = {10.1021/acsnano.9b02818},

year  = {2019},

date = {2019-05-28},

journal = {ACS Nano},

volume = {13},

number = {7},

pages = {7493-7501},

abstract = {We program the optical properties of colloidal Au nanocrystal (NC) assemblies via an unconventional ligand hybridization (LH) strategy to precisely engineer interparticle interactions and design materials with optical properties difficult or impossible to achieve in bulk form. Long-chain hydrocarbon ligands used in NC synthesis are partially exchanged, from 0% to 100%, with compact thiocyanate ligands by controlling the reaction time for exchange. The resulting NC assemblies show transmittance, reflectance, optical permittivity, and direct-current (DC) resistivity that continuously traverse a dielectric-metal transition, providing analog tuning of their physical properties, unlike the digital control realized by complete exchange with ligands of varying length. Exploiting this LH strategy, we create Au NC assemblies that are strong, ultrathin film optical absorbers, as seen by a 6× increase in the extinction of infrared light compared to that in bulk Au thin films and by a temperature rise of 20 °C upon illumination with 808 nm light. Our LH strategy may be applied to the design of materials constructed from NCs of different size, shape, and composition for specific applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zhao2019,

title = {General Synthetic Route to High-Quality Colloidal III–V Semiconductor Quantum Dots Based on Pnictogen Chlorides},

author = {Tianshuo Zhao and Nuri Oh and Davit Jishkariani and Mingliang Zhang and Han Wang and Na Li and Jennifer D. Lee and Chenjie Zeng and Manisha Muduli and Hak-Jong Choi and Dong Su and Christopher B. Murray and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/abs/10.1021/jacs.9b06652},

doi = {10.1021/jacs.9b06652},

year  = {2019},

date = {2019-09-07},

journal = {Journal of the American Chemical Society},

volume = {141},

number = {38},

pages = {15145-15152},

abstract = {The synthesis of colloidal III–V quantum dots (QDs), particularly of the arsenides and antimonides, has been limited by the lack of stable and available group V precursors. In this work, we exploit accessible InCl3- and pnictogen chloride-oleylamine as precursors to synthesize III–V QDs. Through coreduction reactions of the precursors, we achieve size- and stoichiometry-tunable binary InAs and InSb as well as ternary alloy InAs1–xSbx QDs. On the basis of structural, analytical, optical, and electrical characterization of the QDs and their thin-film assemblies, we study the effects of alloying on their particle formation and optoelectronic properties. We introduce a hydrazine-free hybrid ligand-exchange process to improve carrier transport in III–V QD thin films and realize InAs QD field-effect transistors with electron mobility > 5 cm2/(V s). We demonstrate that III–V QD thin films are promising candidate materials for infrared devices and show InAs1–xSbx QD photoconductors with superior short-wavelength infrared (SWIR) photoresponse than those of the binary QD devices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Greybush2019,

title = {Plasmonic Optical and Chiroptical Response of Self-Assembled Au Nanorod Equilateral Trimers},

author = {Nicholas J. Greybush and Victor Pacheco-Peña and Nader Engheta and Christopher B. Murray and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/10.1021/acsnano.8b07619},

doi = {10.1021/acsnano.8b07619},

year  = {2019},

date = {2019-01-10},

journal = {ACS Nano},

volume = {13},

number = {2},

pages = {1617–1624},

abstract = {Assembling metamolecules from anisotropic, shape-engineered nanocrystals provides the opportunity to orchestrate distinct optical responses one nanocrystal at a time. The Au nanorod has long been a structural archetype in plasmonics, but nanorod assemblies have largely been limited to end-to-end or side-to-side arrangements, accessing only a subset of potential metamolecule structures. Here, we employ triangular templates to direct the assembly of Au nanorods along the edges of an equilateral triangle. Using spatially resolved, dark-field scattering spectroscopy in concert with numerical simulation of individual metamolecules, we map the evolution in surface plasmon resonances as we add one, two, and three nanorods to construct triangular nanorod assemblies. The assemblies exhibit rotation- and polarization-dependent hybridized plasmon modes, which are sensitive to variations in nanorod size, position, and orientation that lead to geometrical symmetry breaking. The triangular arrangement of nanorods supports magnetic plasmon modes where electric fields are directed along the perimeter of the triangle, and the magnetic field intensity within the triangle’s open interior is enhanced. Circumferential displacements of the nanorods within the templates impart either a left- or right-handed sense of rotation to the structure, which generates a chiroptical response under unidirectional oblique illumination. Our results represent an important step in realizing and characterizing metamaterial assemblies with “open” structures utilizing anisotropic plasmonic building blocks, with implications for optical magnetic field enhancement and chiral plasmonics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Straus2020,

title = {Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification},

author = {Daniel B Straus and Sebastian Hurtado Parra and Natasha Iotov and Qinghua Zhao and Michael R. Gau and Patrick J. Carroll and James M. Kikkawa and and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/10.1021/acsnano.0c00037},

doi = {10.1021/acsnano.0c00037},

year  = {2020},

date = {2020-03-02},

journal = {ACS Nano},

volume = {14},

number = {3},

pages = {3621–3629},

abstract = {We report a family of two-dimensional hybrid perovskites (2DHPs) based on phenethylammonium lead iodide ((PEA)2PbI4) that show complex structure in their low-temperature excitonic absorption and photoluminescence (PL) spectra as well as hot exciton PL. We replace the 2-position (ortho) H on the phenyl group of the PEA cation with F, Cl, or Br to systematically increase the cation’s cross-sectional area and mass and study changes in the excitonic structure. These single atom substitutions substantially change the observable number of and spacing between discrete resonances in the excitonic absorption and PL spectra and drastically increase the amount of hot exciton PL that violates Kasha’s rule by over an order of magnitude. To fit the progressively larger cations, the inorganic framework distorts and is strained, reducing the Pb–I–Pb bond angles and increasing the 2DHP band gap. Correlation between the 2DHP structure and steady-state and time-resolved spectra suggests the complex structure of resonances arises from one or two manifolds of states, depending on the 2DHP Pb–I–Pb bond angle (as)symmetry, and the resonances within a manifold are regularly spaced with an energy separation that decreases as the mass of the cation increases. The uniform separation between resonances and the dynamics that show excitons can only relax to the next-lowest state are consistent with a vibronic progression caused by a vibrational mode on the cation. These results demonstrate that simple changes to the cation can be used to tailor the properties and dynamics of the confined excitons without directly modifying the inorganic framework.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Straus2019,

title = {Longer Cations Increase Energetic Disorder in Excitonic 2D Hybrid Perovskites},

author = {Daniel B. Straus and Natasha Iotov and Michael R. Gau and Qinghua Zhao and Patrick J. Carroll and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/10.1021/acs.jpclett.9b00247},

doi = {10.1021/acs.jpclett.9b00247},

year  = {2019},

date = {2019-02-26},

journal = {Journal of Physical Chemistry Letters},

volume = {10},

number = {6},

pages = {1198–1205},

abstract = {We synthesize and characterize derivatives of the two-dimensional hybrid perovskite (2DHP) phenethylammonium lead iodide ((PEA)2PbI4) in which the para H on the cation is replaced with F, Cl, CH3, or Br. These substitutions increase the length of the cation but leave the cross-sectional area unchanged, resulting in structurally similar PbI42– frameworks with increasing interlayer spacing. Longer cations result in broader, blue-shifted excitonic absorption spectra with reduced or eliminated structure, indicating greater energetic disorder. Photoluminescence spectra are largely invariant and insensitive to cation length, suggesting polaron formation stabilizes a structural and electronic minimum. Temperature-dependent line width analysis reveals excitons couple to a vibration on the organic framework that is weakly sensitive to these cation substitutions, and Raman spectra and electronic structure calculations support the presence of such a cationic mode. Despite carriers being confined to the inorganic framework, the length of the organic cation alters the optical and electronic properties of 2DHPs.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}