@article{Lynch2024b,

title = {Gate-Tunable Optical Anisotropy in Wafer-Scale, Aligned Carbon Nanotube Films},

author = {Jason Lynch and Evan Smith and Adam Alfieri and Baokun Song and Matthew Klein and Christopher E. Stevens and Cindy Yueli Chen and Chavez FK. Lawrence and Cherie R. Kagan and Honggang Gu and Shiyuan Liu and Lian-Mao Peng and Shivashankar Vangala and Joshua R. Hendrickson and Deep Jariwala},

doi = {10.1038/s41566-024-01504-0},

year  = {2024},

date = {2024-08-14},

urldate = {2024-08-14},

journal = {Nature Photonics},

abstract = {Telecommunications and polarimetry both require the active control of the polarization of light. Currently, this is done by combining intrinsically anisotropic materials with tunable isotropic materials into heterostructures using complicated fabrication techniques owing to the lack of scalable materials that possess both properties. Tunable birefringent and dichromic materials are scarce and rarely available in high-quality thin films over wafer scales. Here we report semiconducting, highly aligned, single-walled carbon nanotubes (SWCNTs) over 4″ wafers with normalized birefringence and dichroism values of 0.09 and 0.58, respectively. The real and imaginary parts of the refractive index of these SWCNT films are tuned by up to 5.9% and 14.3% in the infrared at 2,200 nm and 1,660 nm, respectively, using electrostatic doping. Our results suggest that aligned SWCNTs are among the most anisotropic and tunable optical materials known and open new avenues for their application in integrated photonics and telecommunications.},

keywords = {carbon nanotubes, gate-tunable refractive index, in-plane anisotropy, nanowires, optical properties, thin films},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {One-pot heat-up synthesis of short-wavelength infrared, colloidal InAs quantum dots},

author = {J Lee and T Zhao and S Yang and M Muduli and CB Murray and CR Kagan},

url = {https://pubs.aip.org/aip/jcp/article/160/7/071103/3266823},

doi = {10.1063/5.0187162},

year  = {2024},

date = {2024-02-21},

urldate = {2024-02-21},

journal = {The Journal of Chemical Physics},

volume = {160},

pages = {071103},

abstract = {III–V colloidal quantum dots (QDs) promise Pb and Hg-free QD compositions with which to build short-wavelength infrared (SWIR) optoelectronic devices. However, their synthesis is limited by the availability of group-V precursors with controllable reactivities to prepare monodisperse, SWIR-absorbing III–V QDs. Here, we report a one-pot heat-up method to synthesize ∼8 nm edge length (∼6.5 nm in height) tetrahedral, SWIR-absorbing InAs QDs by increasing the [In3+]:[As3+] ratio introduced using commercially available InCl3 and AsCl3 precursors and by decreasing the concentration and optimizing the volume of the reducing reagent superhydride to control the concentration of In(0) and As(0) intermediates through QD nucleation and growth. InAs QDs are treated with NOBF4, and their deposited films are exchanged with Na2S to yield n-type InAs QD films. We realize the only colloidal InAs QD photoconductors with responsivity at the technologically important wavelength of 1.55 μm.},

keywords = {colloids, nanocrystal, nanocrystal electronics, optical properties, quantum dots, semiconductors, spectroscopy, surface modification, synthesis, TEM, thin films, transport},

pubstate = {published},

tppubtype = {article}

}

@article{Mallavarapu2023,

title = {TiO2 Metasurfaces with Visible Quasi-Guided Mode Resonances via Direct Imprinting of Aqueous Nanocrystal Dispersions},

author = {Akhila Mallavarapu and Chavez FK Lawrence and Brian Huang and Bryan O Torres Maldonado and Paulo Arratia and Cherie R Kagan},

url = {https://pubs.acs.org/doi/abs/10.1021/acsanm.3c03507},

doi = {10.1021/acsanm.3c03507},

year  = {2023},

date = {2023-09-07},

journal = {ACS Applied Nano Materials},

abstract = {We report a room temperature, environmentally benign, water-based, single-step direct nanoimprint process to pattern dielectric metasurfaces using aqueous titanium dioxide (TiO2) nanocrystal (NC) inks, which are free of polymer additives or nonaqueous solvents typically used in nanofabrication. The metasurfaces are composed of TiO2 NC structures with a high refractive index of 1.94 ± 0.02 at 543 nm. TiO2 NC metasurfaces are designed to resonate at visible wavelengths and are fabricated as two-dimensional nanopillar gratings atop waveguides. Guided mode resonances within the waveguide couple to the overlaying gratings and scatter into free space, forming high quality (Q) factor quasi-guided mode (QGM) resonances. Electric and magnetic QGM resonances are observed, and their environmental refractive index sensitivities (S) are measured to be 69.1 and 70.4 nm/RIU, respectively, with a figure of merit (FOM) = Q × S > 3000. The use of water-based inks and the room temperature processing allow integration of TiO2 NC metasurfaces on rigid and flexible, polymeric substrates.},

keywords = {dielectric metasurfaces, ink-lithography, lithography, metasurfaces, nanocrystal, nanoimprinting, optical metamaterials, optical properties, quasi-guided modes, sensors, sustainable manufacturing, TiO2 nanocrystals},

pubstate = {published},

tppubtype = {article}

}

@article{Choi2023,

title = {Surface Engineering of Metal and Semiconductor Nanocrystal Assemblies and Their Optical and Electronic Devices},

author = {Yun Chang Choi and Jaeyoung Lee and Jonah J. Ng and Cherie R. Kagan},

url = {https://doi.org/10.1021/acs.accounts.3c00147},

doi = {10.1021/acs.accounts.3c00147},

year  = {2023},

date = {2023-06-21},

urldate = {2023-06-21},

journal = {Accounts of Chemical Research},

volume = {56},

number = {13},

pages = {1791–1802},

abstract = {Colloidal nanocrystals (NCs) are composed of inorganic cores and organic or inorganic ligand shells and serve as building blocks of NC assemblies. Metal and semiconductor NCs are well known for the size-dependent physical properties of their cores. The large NC surface-to-volume ratio and the space between NCs in assemblies places significant importance on the composition of the NC surface and ligand shell. Nonaqueous colloidal NC syntheses use relatively long organic ligands to control NC size and uniformity during growth and to prepare stable NC dispersions. However, these ligands create large interparticle distances that dilute the metal and semiconductor NC properties of their assemblies. In this Account, we describe postsynthesis chemical treatments to engineer the NC surface and design the optical and electronic properties of NC assemblies. In metal NC assemblies, compact ligand exchange reduces the interparticle distance and drives an insulator-to-metal transition tuning the dc resistivity over a 1010 range and the real part of the optical dielectric function from positive to negative across the visible-to-IR region. Juxtaposing NC and bulk metal thin films in bilayers allows the differential chemical and thermal addressability of the NC surface to be exploited in device fabrication. Ligand exchange and thermal annealing densifies the NC layer, creating interfacial misfit strain that triggers folding of the bilayers and is used to fabricate, with only one lithography step, large-area 3D chiral metamaterials. In semiconductor NC assemblies, chemical treatments such as ligand exchange, doping, and cation exchange control the interparticle distance and composition to add impurities, tailor stoichiometry, or make entirely new compounds. These treatments are employed in longer studied II–VI and IV–VI materials and are being developed as interest in III–V and I–III–VI2 NC materials grows. NC surface engineering is used to design NC assemblies with tailored carrier energy, type, concentration, mobility, and lifetime. Compact ligand exchange increases the coupling between NCs but can introduce intragap states that scatter and reduce the lifetime of carriers. Hybrid ligand exchange with two different chemistries can enhance the mobility-lifetime product. Doping increases carrier concentration, shifts the Fermi energy, and increases carrier mobility, creating n- and p-type building blocks for optoelectronic and electronic devices and circuits. Surface engineering of semiconductor NC assemblies is also important to modify device interfaces to allow the stacking and patterning of NC layers and to realize excellent device performance. It is used to construct NC-integrated circuits, exploiting the library of metal, semiconductor, and insulator NCs, to achieve all-NC, solution-fabricated transistors.},

keywords = {gold, ligand exchange, ligands, nanocrystal, nanoparticle assembly, Noble metal nanoparticles, optical metamaterials, optical properties, quantum dots, semiconductors, surface modification},

pubstate = {published},

tppubtype = {article}

}

@article{Cai2023,

title = {Open- and Close-Packed, Shape-engineered Polygonal Nanoparticle Metamolecules with Tailorable Fano Resonances},

author = {Yi-Yu Cai and Asma Fallah and Shengsong Yang and Yun Chang Choi and Jun Xu and Aaron Stein and James M. Kikkawa and Christopher B. Murray and Nader Engheta and Cherie R. Kagan},

url = { https://doi.org/10.1002/adma.202301323},

doi = {10.1002/adma.202301323},

year  = {2023},

date = {2023-05-11},

urldate = {2023-05-11},

journal = {Advanced Materials},

abstract = {A top-down lithographic patterning and deposition process is reported for producing nanoparticles (NPs) with well-defined sizes, shapes, and compositions that are often not accessible by wet-chemical synthetic methods. These NPs are ligated and harvested from the substrate surface to prepare colloidal NP dispersions. Using a template-assisted assembly technique, fabricated NPs are driven by capillary forces to assemble into size- and shape-engineered templates and organize into open or close-packed multi-NP structures or NP metamolecules. The sizes and shapes of the NPs and of the templates control the NP number, coordination, interparticle gap size, disorder, and location of defects such as voids in the NP metamolecules. The plasmonic resonances of polygonal-shaped Au NPs are exploited to correlate the structure and optical properties of assembled NP metamolecules. Comparing open- and close-packed architectures highlights that introduction of a center NP to form closed-packed assemblies supports collective interactions, altering magnetic optical modes and multipolar interactions in Fano resonances. Decreasing the distance between NPs strengthens the plasmonic coupling, and the structural symmetries of the NP metamolecules determine the orientation-dependent scattering response.},

keywords = {nanoparticle assembly, optical properties, plasmonic, templated assembly},

pubstate = {published},

tppubtype = {article}

}

@article{Thompson2023,

title = {Red Emission from Copper-Vacancy Color Centers in Zinc Sulfide Colloidal Nanocrystals},

author = {Sarah M. Thompson and Cüneyt Şahin and Shengsong Yang and Michael E. Flatté and Christopher B. Murray and Lee C. Bassett and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/full/10.1021/acsnano.3c00191

https://arxiv.org/abs/2301.04223},

doi = {10.1021/acsnano.3c00191},

year  = {2023},

date = {2023-03-09},

urldate = {2023-03-09},

journal = {ACS Nano},

volume = {17},

number = {6},

pages = {5963-5973},

abstract = {Copper-doped zinc sulfide (ZnS:Cu) exhibits down-conversion luminescence in the UV, visible, and IR regions of the electromagnetic spectrum; the visible red, green, and blue emission is referred to as R-Cu, G-Cu, and B-Cu, respectively. The sub-bandgap emission arises from optical transitions between localized electronic states created by point defects, making ZnS:Cu a prolific phosphor material and an intriguing candidate material for quantum information science, where point defects excel as single-photon sources and spin qubits. Colloidal nanocrystals (NCs) of ZnS:Cu are particularly interesting as hosts for the creation, isolation, and measurement of quantum defects, since their size, composition, and surface chemistry can be precisely tailored for bio-sensing and opto-electronic applications. Here, we present a method for synthesizing colloidal ZnS:Cu NCs that emit primarily R-Cu, which has been proposed to arise from the Cu_{Zn}-V_{S} complex, an impurity-vacancy point defect structure analogous to well-known quantum defects in other materials that produce favorable optical and spin dynamics. First principles calculations confirm the thermodynamic stability and electronic structure of Cu_{Zn}-V_{S}. Temperature- and time-dependent optical properties of ZnS:Cu NCs show blueshifting luminescence and an anomalous plateau in the intensity dependence as temperature is increased from 19 K to 290 K, for which we propose an empirical dynamical model based on thermally-activated coupling between two manifolds of states inside the ZnS bandgap. Understanding of R-Cu emission dynamics, combined with a controlled synthesis method for obtaining R-Cu centers in colloidal NC hosts, will greatly facilitate the development of Cu_{Zn}-V_{S} and related complexes as quantum point defects in ZnS.},

keywords = {color centers, doping, impurities, nanocrystal, optical properties, quantum information science, time-resolved photoluminescence, transition metals, zinc sulfide},

pubstate = {published},

tppubtype = {article}

}

@article{Neuhaus2023,

title = {Frequency Stabilization and Optically Tunable Lasing in Colloidal Quantum Dot Superparticles},

author = {Steven J. Neuhaus and Emanuele Marino and Christopher B. Murray and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/full/10.1021/acs.nanolett.2c04498},

doi = {10.1021/acs.nanolett.2c04498},

year  = {2023},

date = {2023-01-05},

urldate = {2023-01-05},

journal = {Nano Letters},

volume = {23},

issue = {2},

pages = {645–651},

abstract = {Self-assembled superparticles composed of colloidal quantum dots establish microsphere cavities that support optically pumped lasing from whispering gallery modes. Here, we report on the time- and excitation fluence-dependent lasing properties of CdSe/CdS quantum dot superparticles. Spectra collected under constant photoexcitation reveal that the lasing modes are not temporally stable but instead blue-shift by more than 30 meV over 15 min. To counter this effect, we establish a high-fluence light-soaking protocol that reduces this blue-shift by more than an order of magnitude to 1.7 ± 0.5 meV, with champion superparticles displaying mode blue-shifts of <0.5 meV. Increasing the pump fluence allows for optically controlled, reversible, color-tunable red-to-green lasing. Combining these two paradigms suggests that quantum dot superparticles could serve in applications as low-cost, robust, solution-processable, tunable microlasers.},

keywords = {lasers, lasing, nanocrystal, optical properties, optical stability, quantum dots, superparticle, supraparticle, tunable laser},

pubstate = {published},

tppubtype = {article}

}

@article{Gabinet2022,

title = {Magnetic Field Alignment and Optical Anisotropy of MoS2 Nanosheets Dispersed in a Liquid Crystal Polymer},

author = {Uri R. Gabinet and Changyeon Lee and Na Kyung Kim and Martin Hulman and Sarah M. Thompson and Cherie R. Kagan and Chinedum O. Osuji},

url = {https://pubs.acs.org/doi/full/10.1021/acs.jpclett.2c01819},

doi = {10.1021/acs.jpclett.2c01819},

year  = {2022},

date = {2022-08-19},

urldate = {2022-08-19},

journal = {The Journal of Physical Chemistry Letters},

volume = {13},

issue = {34},

pages = {7994–8001},

abstract = {Molybdenum disulfide (MoS2) nanosheets exhibit anisotropic optical and electronic properties, stemming from their shape and electronic structure. Unveiling this anisotropy for study and usage in materials and devices requires the ability to control the orientation of dispersed nanosheets, but to date this has proved a challenging proposition. Here, we demonstrate magnetic field driven alignment of MoS2 nanosheets in a liquid crystal (LC) polymer and unveil the optical properties of the resulting anisotropic assembly. Nanosheet optical anisotropy is observed spectroscopically by Raman and direction-dependent photoluminescence (PL) measurements. Resulting data indicate significantly lower PL emission due to optical excitation with electric field oscillation out of plane, parallel to the MoS2c-axis, than that associated with perpendicular excitation, with the dichroic ratio Iperp/Ipar = 3. The approach developed here provides a useful route to elucidate anisotropic optical properties of MoS2 nanosheets and to utilize such properties in new materials and devices.},

keywords = {2D-materials, liquid chromatography, magnetic properties, optical properties, scattering},

pubstate = {published},

tppubtype = {article}

}

@article{Marino2022,

title = {Nanocrystal Superparticles with Whispering-Gallery Modes Tunable through Chemical and Optical Triggers},

author = {Emanuele Marino and Harshit Bharti and Jun Xu and Cherie R. Kagan and Christopher B. Murray},

url = {https://pubs.acs.org/doi/full/10.1021/acs.nanolett.2c01011},

doi = {10.1021/acs.nanolett.2c01011},

year  = {2022},

date = {2022-06-01},

journal = {Nano Letters},

volume = {22},

issue = {12},

pages = {4765–4773},

abstract = {Whispering-gallery microresonators have the potential to become the building blocks for optical circuits. However, encoding information in an optical signal requires on-demand tuning of optical resonances. Tuning is achieved by modifying the cavity length or the refractive index of the microresonator. Due to their solid, nondeformable structure, conventional microresonators based on bulk materials are inherently difficult to tune. In this work, we fabricate irreversibly tunable optical microresonators by using semiconductor nanocrystals. These nanocrystals are first assembled into colloidal spherical superparticles featuring whispering-gallery modes. Exposing the superparticles to shorter ligands changes the nanocrystal surface chemistry, decreasing the cavity length of the microresonator by 20% and increasing the refractive index by 8.2%. Illuminating the superparticles with ultraviolet light initiates nanocrystal photo-oxidation, providing an orthogonal channel to decrease the refractive index of the microresonator in a continuous fashion. Through these approaches, we demonstrate optical microresonators tunable by several times their free spectral range.},

keywords = {cavities, CdSe, emulsions, ligands, optical properties},

pubstate = {published},

tppubtype = {article}

}

@article{Chen2015,

title = {Large-Area Nanoimprinted Colloidal Au Nanocrystal-Based Nanoantennas for Ultrathin Polarizing Plasmonic Metasurfaces},

author = {Wenxiang Chen and Mykhailo Tymchenko∥ and Prashanth Gopalan and Xingchen Ye and Yaoting Wu and Mingliang Zhang and Christopher B. Murray and Andrea Alu and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b02647},

doi = {10.1021/acs.nanolett.5b02647},

year  = {2015},

date = {2015-07-10},

urldate = {2015-07-10},

journal = {Nano Letters},

volume = {15},

number = {8},

issue = {8},

pages = {5254–5260},

abstract = {We report a low-cost, large-area fabrication process using solution-based nanoimprinting and compact ligand exchange of colloidal Au nanocrystals to define anisotropic, subwavelength, plasmonic nanoinclusions for optical metasurfaces. Rod-shaped, Au nanocrystal-based nanoantennas possess strong, localized, plasmonic resonances able to control polarization. We fabricate metasurfaces from rod-shaped nanoantennas tailored in size and spacing to demonstrate Au nanocrystal-based quarter-wave plates that operate with extreme bandwidths and provide high polarization conversion efficiencies in the near-to-mid infrared.},

keywords = {gold, nanocrystal, nanoimprinting, optical metamaterials, optical properties, plasmonic},

pubstate = {published},

tppubtype = {article}

}

@article{Diroll2015,

title = {Smectic Nanorod Superlattices Assembled on Liquid Subphases: Structure, Orientation, Defects, and Optical Polarization},

author = {Benjamin T. Diroll and Nicholas J. Greybush and Cherie R. Kagan and Christopher B. Murray},

url = {https://pubs.acs.org/doi/abs/10.1021/acs.chemmater.5b00355},

doi = {10.1021/acs.chemmater.5b00355},

year  = {2015},

date = {2015-03-10},

urldate = {2015-03-10},

journal = {Chemistry of Materials},

volume = {27},

issue = {8},

pages = {2998–3008},

abstract = {Directing the orientation of anisotropic nanocrystal assemblies is important for harnessing the shape-dependent properties of nanocrystal solids in devices. We control the orientation of smectic B superlattices of CdSe/CdS dot-in-rod nanocrystals through assembly on different polar interfaces and quantify the superlattice orientation through correlated small- and wide-angle grazing-incidence diffraction. Small-angle scattering is used to determine the phase of the nanorod superlattices and their preferential growth directions from the subphase. Wide-angle diffraction is used to quantify the orientations of nanorods within the superlattices and with respect to the substrate. Not only are the nanorod long axes aligned within the structures, but truncation of the short axes also coaligns the crystal axes of the nanorods with the zone axes in assembled smectic B crystals. Three dimensional orientational alignment of nanocrystals in superlattices is highly desirable in device applications. Depending on the subphase used for self-assembly, the films range from nearly quantitative vertical to horizontal alignment. Controlling for other variables, we find that the surface tension of the subphase is strongly correlated with the orientational ordering of the nanorod superlattices. The microstructure of nanorod superlattices shows many classic defects of atomic and liquid crystalline systems. The nature of defect structures supports a mechanism of nuclei formation on the subphase–solvent interface rather than in solution. Last, we demonstrate the relationship between optical absorption polarization and superlattice structure using correlated optical spectroscopy and electron microscopy.},

keywords = {defects, liquid crystals, nanorods, optical properties, self-assembly, superlattices},

pubstate = {published},

tppubtype = {article}

}

@article{Goodwin2014,

title = {Effects of Post-Synthesis Processing on CdSe Nanocrystals and Their Solids: Correlation between Surface Chemistry and Optoelectronic Properties},

author = {E. D. Goodwin and Benjamin T. Diroll and Soong Ju Oh and Taejong Paik and Christopher B. Murray and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/full/10.1021/jp5076912},

doi = {10.1021/jp5076912},

year  = {2014},

date = {2014-10-21},

urldate = {2014-10-21},

journal = {The Journal of Physical Chemistry C},

volume = {118},

number = {46},

pages = {27097–27105},

abstract = {In this work, we report the effects on CdSe nanocrystal (NC) surface chemistry of acetone and methanol when used as the antisolvents for NC washing and as the solvents for ligand exchange of NC solids with ammonium thiocyanate (NH4SCN). We find that NCs washed with methanol have significantly fewer remaining organic ligands and lower photoluminescence quantum yield than those washed with acetone. When used as the ligand exchange solvent, methanol leaves more organic ligands and introduces fewer bound thiocyanates on the NC surface than when acetone is used. We demonstrate the effect of these different surface chemistries on NC solid optoelectronic properties through photoconductivity measurements, showing a greater photocurrent in NC solids with greater organic ligand coverage. We also show that NC washing with methanol or ligand exchange with NH4SCN in methanol removes a significant number of surface Cd atoms from the NCs, creating Cd vacancies that act as traps for recombination. Independent of the wash and exchange process, the NC surface may be repaired by introducing CdCl2 to the NC surface, enhancing the measured photocurrent.},

keywords = {CdSe, nanocrystal, nanocrystal electronics, optical properties, optical stability, semiconductors, surface modification, transport},

pubstate = {published},

tppubtype = {article}

}

@article{Diroll2014b,

title = {Synthesis of N-Type Plasmonic Oxide Nanocrystals and the Optical and Electrical Characterization of their Transparent Conducting Films},

author = {Benjamin T. Diroll and Thomas R. Gordon and E. Ashley Gaulding and Dahlia R. Klein and Taejong Paik and Hyeong Jin Yun and E.D. Goodwin and Divij Damodhar and Cherie R. Kagan and Christopher B. Murray},

url = {https://pubs.acs.org/doi/full/10.1021/cm5018823},

doi = {10.1021/cm5018823},

year  = {2014},

date = {2014-07-18},

journal = {Chemistry of Materials},

volume = {26},

number = {15},

pages = {4579–4588},

abstract = {We present a general synthesis for a family of n-type transparent conducting oxide nanocrystals through doping with aliovalent cations. These monodisperse nanocrystals exhibit localized surface plasmon resonances tunable in the mid- and near-infrared with increasing dopant concentration. We employ a battery of electrical measurements to demonstrate that the plasmonic resonance in isolated particles is consistent with the electronic properties of oxide nanocrystal thin films. Hall and Seebeck measurements show that the particles form degenerately doped n-type solids with free electron concentrations in the range of 1019 to 1021 cm–3. These heavily doped oxide nanocrystals are used as the building blocks of conductive, n-type thin films with high visible light transparency.},

keywords = {doping, nanocrystal, nanocrystal electronics, optical properties, plasmonic, thin films},

pubstate = {published},

tppubtype = {article}

}