ABSTRACT We synthesized CdSe/CdS giant-shell nanocrystals, with a CdSe core diameter between 2.8 ... more ABSTRACT We synthesized CdSe/CdS giant-shell nanocrystals, with a CdSe core diameter between 2.8 nm and 5.5 nm{,} and a CdS shell thickness of up to 7-8 nm (equivalent to about 20 monolayers of CdS). Both the core and shell have a wurtzite crystal structure, yielding epitaxial growth of the shell and nearly defect-free crystals. As a result, the photoluminescence (PL) quantum efficiency (QE) is as high as 90%. Quantitative PL measurements at various excitation wavelengths allow us to separate the nonradiative decay into contributions from interface and surface trapping, giving us pathways for future optimization of the structure. In addition, the NCs do not blink, and the giant shell and concurring strong electron delocalization efficiently suppress Auger recombination, yielding a biexciton lifetime of about 15 ns. The corresponding biexciton PL QE equals 11% in 5.5/18.1 nm CdSe/CdS. Variable-temperature time-resolved PL and PL under magnetic fields further reveal that the emission at cryogenic temperature originates from a negative trion-state, in agreement with other CdSe/CdS giant-shell systems reported in the literature.
Core–shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. T... more Core–shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. Their solution-phase synthesis is based on surface-stabilizing ligands that make them soluble in organic solvents, like toluene or chloroform. However, solubility of these materials in water provides many advantages, such as additional process routes and easier handling. So far, solubilization of CdSe/CdS nanocrystals in water that avoids detrimental effects on the luminescent properties poses a major challenge. This work demonstrates how core–shell CdSe/CdS quantum dot-in-rods can be transferred into water using a ligand exchange method employing mercaptopropionic acid (MPA). Key to maintaining the light-emitting properties is an enlarged CdS rod diameter, which prevents potential surface defects formed during the ligand exchange from affecting the photophysics of the dot-in-rods. Films made from water-soluble dot-in-rods show amplified spontaneous emission (ASE) with a similar threshold (130 μJ/cm2) as the pristine material (115 μJ/cm2). To demonstrate feasibility for lasing applications, self-assembled microlasers are fabricated via the “coffee-ring effect” that display single-mode operation and a very low threshold of ∼10 μJ/cm2. The performance of these microlasers is enhanced by the small size of MPA ligands, enabling a high packing density of the dot-in-rods.
The next generation of radiation detectors used in security scanning, medical imaging, and high e... more The next generation of radiation detectors used in security scanning, medical imaging, and high energy physics applications depend on understanding the physical mechanisms that limit intrinsic energy resolution. Nonproportionality between electron energy response and the initial energy of a stopping electron is a significant cause of the degradation of energy resolution. The physical origin of nonproportional response is generally regarded to start with nonlinear quenching, proportional to 2nd or 3rd power of local excitation density.
Abstract We have previously described a numerical model for carrier diffusion and nonlinear quenc... more Abstract We have previously described a numerical model for carrier diffusion and nonlinear quenching in the track of an electron in a scintillator. Significant inequality of electron and hole mobilities predicts a characteristic “hump” in the light yield vs gamma energy, whereas low mobility of either or both carriers accentuates the universal roll-off due to nonlinear quenching at low gamma energy (high dE/dx).
Abstract Recent years have seen significant advances in both theoretically understanding and math... more Abstract Recent years have seen significant advances in both theoretically understanding and mathematically modeling the underlying causes of scintillator non-proportionality. The core cause is that the interaction of radiation with matter invariably leads to a non-uniform ionization density in the scintillator, coupled with the fact that the light yield depends on the ionization density.
Intrinsic energy resolution in γ-radiation detectors is limited by nonlinear quenching that varie... more Intrinsic energy resolution in γ-radiation detectors is limited by nonlinear quenching that varies as the second or third power of local excitation density. Using a numerical model for local light yield depending on measured quenching rates, kinetic order, and carrier mobilities, we employ Monte Carlo simulations of energy deposited at each local excitation density to calculate electron energy response that can be directly compared with Compton coincidence and K-dip experiments. Agreement is found for NaI:Tl and SrI2:Eu using thermalized carrier diffusion and linear quenched fraction deduced from total light yield. This lays the groundwork for testing refinements with recent hot-electron extensions of the model.(Received June 22 2012)(Accepted October 19 2012)
Stopping of an energetic electron produces a track of high excitation density, especially near it... more Stopping of an energetic electron produces a track of high excitation density, especially near its end, and consequent high radial concentration gradient. The effect of high excitation density in promoting nonlinear quenching is generally understood to be a root cause of nonproportionality in scintillators. However, quantitative data on the kinetic rates of nonlinear quenching processes in scintillators are scarce. We report experimental measurements of second-order dipole–dipole rate constants governing the main nonlinear quenching channel in CsI, CsI:Tl, NaI, and NaI:Tl. We also show that the second of the extreme conditions in a track, i.e., radial concentration gradient, gives rise to fast (≤picoseconds) diffusion phenomena which act both as a competitor in reducing excitation density during the relevant time of nonlinear quenching, and as a determiner of branching between independent and paired carriers, where the branching ratio changes with dE/dx along the primary electron track. To investigate the interplay of these phenomena in determining nonproportionality of light yield, we use experimentally measured rate constants and mobilities in CsI and NaI to carry out quantitative modeling of diffusion, drift, and nonlinear quenching evaluated spatially and temporally within an electron track which is assumed cylindrical Gaussian in this version of the model.
Abstract-We report picosecond time-resolved measurements of optical absorption induced by a sub-p... more Abstract-We report picosecond time-resolved measurements of optical absorption induced by a sub-picosecond pulse of light producing two-photon bandgap excitation of Csl and CsI: Tl at room temperature. The transient spectrum of undoped Csl reveals for the first time strong infrared absorption rising through the 0.8-eV limit of present measurements. We suggest that this infrared band is due to transitions of the bound electron in the off-center self-trapped exciton (STE), implying that there should be a band deeper in the infrared ...
Numerical modeling and comparison to experiment in the materials for which suitable parameters ha... more Numerical modeling and comparison to experiment in the materials for which suitable parameters have been measured confirm that three of the most important material parameters for predicting proportionality and the related host-dependent light yield (LY) of scintillators are (i) the carrier diffusion coefficients (including hole self-trapping if present, and hot-electron diffusion if unthermalized), (ii) the kinetic order and associated rate constant of nonlinear quenching, and (iii) deep-trapping probability. Thermalized carrier diffusion appears sufficient to describe the main trends in oxides and semiconductors. For heavier halide hosts, it appears necessary to take account of hot-electron diffusion to explain several important host-dependent trends.
The dipole-dipole and free-carrier Auger quenching processes that are generally regarded to be at... more The dipole-dipole and free-carrier Auger quenching processes that are generally regarded to be at the root of nonproportionality depend respectively on the 4th or 6th power of the electron track radius if modeled as cylindrical. In an initial time interval τ when nonlinear quenching and diffusion compete to reduce the density of excited states, the track radius expands as (Deffτ)1/2 where Deff is the effective diffusion coefficient for the mixture of excitons and charge carriers. The range of Deff across semiconductor and scintillator radiation detectors is large, illustrated by 8 decades between mobilities of self-trapped holes in CsI:Tl and holes in high purity Ge. We present the functional form of nonlinear quenching predicted by diffusive track dilution and show that the simple model provides a surprisingly good fit of empirical nonproportionality across a wide range of semiconductor and oxide radiation detectors. We also show how diffusion drives nonlinear branching between excitons and free carriers in the track when electron and hole mobilities are unequal, and that this nonlinear branching coupled with linear trapping on defects can produce the "halide hump" seen in electron yield data for activated halide scintillators. Picosecond time-resolved spectroscopy in alkali halides, as well as quantitative comparison of recently measured 2nd order quenching rate constants K2 and results of K-dip spectroscopy, provide experimental benchmarks for consideration of carrier thermalization and the initial track or cluster radius r0 from which (nearly thermalized) diffusion is assumed to commence. The ratio of initial rate of 2nd order quenching to that of dilution by diffusion in a cylindrical track is proportional to K2/Deff and does not depend on r0 in lowest order; however, the absolute rates of both processes decrease with increasing r0.
Elimination of ghosting artifacts from wavelength-shifting fiber neutron detectors Rev. Sci. Inst... more Elimination of ghosting artifacts from wavelength-shifting fiber neutron detectors Rev. Sci. Instrum. 84, 013308 (2013) An apparatus for studying scintillator properties at high isostatic pressures Rev. Sci. Instrum. 84, 015109 (2013) Digital discrimination of neutrons and gamma-rays in organic scintillation detectors using moment analysis Rev. Sci. Instrum. 83, 093507 (2012) High-resolution spectroscopy used to measure inertial confinement fusion neutron spectra on Omega (invited) Rev. Sci. Instrum. 83, 10D919 Testing a new NIF neutron time-of-flight detector with a bibenzyl scintillator on OMEGA Rev. Sci. Instrum. 83, 10D309 (2012) Additional information on J. Appl. Phys.
Time-dependent radial diffusion and drift are modeled in the high carrier concentration gradient ... more Time-dependent radial diffusion and drift are modeled in the high carrier concentration gradient characteristic of electron tracks in scintillators and other radiation detector materials. As expected, the lower mobility carrier (typically the hole) controls the ambipolar diffusion. Carrier separation when electron and hole mobilities are unequal produces a built-in radial electric field near the track analogous to an n-intrinsic semiconductor junction. The diffusion is shown to have significant effects on both the low dE/dx and high dE/dx ends of ...
Nonproportionality of scintillator light yield\ textit {vs} particle energy is a significant issu... more Nonproportionality of scintillator light yield\ textit {vs} particle energy is a significant issue in new-generation radiation analyzers for homeland security, medical, and astrophysical applications. To study kinetics of quenching at high excitation densities characteristic of electron track ends, we have used 0.3 ps pulses of 5.9-eV light to excite up to 5 x 10$^{20} $ eh/cm $^{3} $ in CsI. Streak camera luminescence and transient absorption data show high-density eh pair excitation yielding partially quenched self-trapped exciton (STE) ...
abstract The effect of high excitation density in promoting nonlinear quenching that is 2 nd or 3... more abstract The effect of high excitation density in promoting nonlinear quenching that is 2 nd or 3 rd order in electron-hole density is generally understood to be a root cause of nonproportionality in scintillators. We report and discuss quantitative data on just how fast these nonlinear channels are in specific cases. Kinetic rate constants for the creation of excitons from electrons and holes and for their quenching by dipole-dipole transfer have been measured in CsI and NaI. We show in addition that the strong radial concentration ...
ABSTRACT We synthesized CdSe/CdS giant-shell nanocrystals, with a CdSe core diameter between 2.8 ... more ABSTRACT We synthesized CdSe/CdS giant-shell nanocrystals, with a CdSe core diameter between 2.8 nm and 5.5 nm{,} and a CdS shell thickness of up to 7-8 nm (equivalent to about 20 monolayers of CdS). Both the core and shell have a wurtzite crystal structure, yielding epitaxial growth of the shell and nearly defect-free crystals. As a result, the photoluminescence (PL) quantum efficiency (QE) is as high as 90%. Quantitative PL measurements at various excitation wavelengths allow us to separate the nonradiative decay into contributions from interface and surface trapping, giving us pathways for future optimization of the structure. In addition, the NCs do not blink, and the giant shell and concurring strong electron delocalization efficiently suppress Auger recombination, yielding a biexciton lifetime of about 15 ns. The corresponding biexciton PL QE equals 11% in 5.5/18.1 nm CdSe/CdS. Variable-temperature time-resolved PL and PL under magnetic fields further reveal that the emission at cryogenic temperature originates from a negative trion-state, in agreement with other CdSe/CdS giant-shell systems reported in the literature.
Core–shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. T... more Core–shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. Their solution-phase synthesis is based on surface-stabilizing ligands that make them soluble in organic solvents, like toluene or chloroform. However, solubility of these materials in water provides many advantages, such as additional process routes and easier handling. So far, solubilization of CdSe/CdS nanocrystals in water that avoids detrimental effects on the luminescent properties poses a major challenge. This work demonstrates how core–shell CdSe/CdS quantum dot-in-rods can be transferred into water using a ligand exchange method employing mercaptopropionic acid (MPA). Key to maintaining the light-emitting properties is an enlarged CdS rod diameter, which prevents potential surface defects formed during the ligand exchange from affecting the photophysics of the dot-in-rods. Films made from water-soluble dot-in-rods show amplified spontaneous emission (ASE) with a similar threshold (130 μJ/cm2) as the pristine material (115 μJ/cm2). To demonstrate feasibility for lasing applications, self-assembled microlasers are fabricated via the “coffee-ring effect” that display single-mode operation and a very low threshold of ∼10 μJ/cm2. The performance of these microlasers is enhanced by the small size of MPA ligands, enabling a high packing density of the dot-in-rods.
The next generation of radiation detectors used in security scanning, medical imaging, and high e... more The next generation of radiation detectors used in security scanning, medical imaging, and high energy physics applications depend on understanding the physical mechanisms that limit intrinsic energy resolution. Nonproportionality between electron energy response and the initial energy of a stopping electron is a significant cause of the degradation of energy resolution. The physical origin of nonproportional response is generally regarded to start with nonlinear quenching, proportional to 2nd or 3rd power of local excitation density.
Abstract We have previously described a numerical model for carrier diffusion and nonlinear quenc... more Abstract We have previously described a numerical model for carrier diffusion and nonlinear quenching in the track of an electron in a scintillator. Significant inequality of electron and hole mobilities predicts a characteristic “hump” in the light yield vs gamma energy, whereas low mobility of either or both carriers accentuates the universal roll-off due to nonlinear quenching at low gamma energy (high dE/dx).
Abstract Recent years have seen significant advances in both theoretically understanding and math... more Abstract Recent years have seen significant advances in both theoretically understanding and mathematically modeling the underlying causes of scintillator non-proportionality. The core cause is that the interaction of radiation with matter invariably leads to a non-uniform ionization density in the scintillator, coupled with the fact that the light yield depends on the ionization density.
Intrinsic energy resolution in γ-radiation detectors is limited by nonlinear quenching that varie... more Intrinsic energy resolution in γ-radiation detectors is limited by nonlinear quenching that varies as the second or third power of local excitation density. Using a numerical model for local light yield depending on measured quenching rates, kinetic order, and carrier mobilities, we employ Monte Carlo simulations of energy deposited at each local excitation density to calculate electron energy response that can be directly compared with Compton coincidence and K-dip experiments. Agreement is found for NaI:Tl and SrI2:Eu using thermalized carrier diffusion and linear quenched fraction deduced from total light yield. This lays the groundwork for testing refinements with recent hot-electron extensions of the model.(Received June 22 2012)(Accepted October 19 2012)
Stopping of an energetic electron produces a track of high excitation density, especially near it... more Stopping of an energetic electron produces a track of high excitation density, especially near its end, and consequent high radial concentration gradient. The effect of high excitation density in promoting nonlinear quenching is generally understood to be a root cause of nonproportionality in scintillators. However, quantitative data on the kinetic rates of nonlinear quenching processes in scintillators are scarce. We report experimental measurements of second-order dipole–dipole rate constants governing the main nonlinear quenching channel in CsI, CsI:Tl, NaI, and NaI:Tl. We also show that the second of the extreme conditions in a track, i.e., radial concentration gradient, gives rise to fast (≤picoseconds) diffusion phenomena which act both as a competitor in reducing excitation density during the relevant time of nonlinear quenching, and as a determiner of branching between independent and paired carriers, where the branching ratio changes with dE/dx along the primary electron track. To investigate the interplay of these phenomena in determining nonproportionality of light yield, we use experimentally measured rate constants and mobilities in CsI and NaI to carry out quantitative modeling of diffusion, drift, and nonlinear quenching evaluated spatially and temporally within an electron track which is assumed cylindrical Gaussian in this version of the model.
Abstract-We report picosecond time-resolved measurements of optical absorption induced by a sub-p... more Abstract-We report picosecond time-resolved measurements of optical absorption induced by a sub-picosecond pulse of light producing two-photon bandgap excitation of Csl and CsI: Tl at room temperature. The transient spectrum of undoped Csl reveals for the first time strong infrared absorption rising through the 0.8-eV limit of present measurements. We suggest that this infrared band is due to transitions of the bound electron in the off-center self-trapped exciton (STE), implying that there should be a band deeper in the infrared ...
Numerical modeling and comparison to experiment in the materials for which suitable parameters ha... more Numerical modeling and comparison to experiment in the materials for which suitable parameters have been measured confirm that three of the most important material parameters for predicting proportionality and the related host-dependent light yield (LY) of scintillators are (i) the carrier diffusion coefficients (including hole self-trapping if present, and hot-electron diffusion if unthermalized), (ii) the kinetic order and associated rate constant of nonlinear quenching, and (iii) deep-trapping probability. Thermalized carrier diffusion appears sufficient to describe the main trends in oxides and semiconductors. For heavier halide hosts, it appears necessary to take account of hot-electron diffusion to explain several important host-dependent trends.
The dipole-dipole and free-carrier Auger quenching processes that are generally regarded to be at... more The dipole-dipole and free-carrier Auger quenching processes that are generally regarded to be at the root of nonproportionality depend respectively on the 4th or 6th power of the electron track radius if modeled as cylindrical. In an initial time interval τ when nonlinear quenching and diffusion compete to reduce the density of excited states, the track radius expands as (Deffτ)1/2 where Deff is the effective diffusion coefficient for the mixture of excitons and charge carriers. The range of Deff across semiconductor and scintillator radiation detectors is large, illustrated by 8 decades between mobilities of self-trapped holes in CsI:Tl and holes in high purity Ge. We present the functional form of nonlinear quenching predicted by diffusive track dilution and show that the simple model provides a surprisingly good fit of empirical nonproportionality across a wide range of semiconductor and oxide radiation detectors. We also show how diffusion drives nonlinear branching between excitons and free carriers in the track when electron and hole mobilities are unequal, and that this nonlinear branching coupled with linear trapping on defects can produce the "halide hump" seen in electron yield data for activated halide scintillators. Picosecond time-resolved spectroscopy in alkali halides, as well as quantitative comparison of recently measured 2nd order quenching rate constants K2 and results of K-dip spectroscopy, provide experimental benchmarks for consideration of carrier thermalization and the initial track or cluster radius r0 from which (nearly thermalized) diffusion is assumed to commence. The ratio of initial rate of 2nd order quenching to that of dilution by diffusion in a cylindrical track is proportional to K2/Deff and does not depend on r0 in lowest order; however, the absolute rates of both processes decrease with increasing r0.
Elimination of ghosting artifacts from wavelength-shifting fiber neutron detectors Rev. Sci. Inst... more Elimination of ghosting artifacts from wavelength-shifting fiber neutron detectors Rev. Sci. Instrum. 84, 013308 (2013) An apparatus for studying scintillator properties at high isostatic pressures Rev. Sci. Instrum. 84, 015109 (2013) Digital discrimination of neutrons and gamma-rays in organic scintillation detectors using moment analysis Rev. Sci. Instrum. 83, 093507 (2012) High-resolution spectroscopy used to measure inertial confinement fusion neutron spectra on Omega (invited) Rev. Sci. Instrum. 83, 10D919 Testing a new NIF neutron time-of-flight detector with a bibenzyl scintillator on OMEGA Rev. Sci. Instrum. 83, 10D309 (2012) Additional information on J. Appl. Phys.
Time-dependent radial diffusion and drift are modeled in the high carrier concentration gradient ... more Time-dependent radial diffusion and drift are modeled in the high carrier concentration gradient characteristic of electron tracks in scintillators and other radiation detector materials. As expected, the lower mobility carrier (typically the hole) controls the ambipolar diffusion. Carrier separation when electron and hole mobilities are unequal produces a built-in radial electric field near the track analogous to an n-intrinsic semiconductor junction. The diffusion is shown to have significant effects on both the low dE/dx and high dE/dx ends of ...
Nonproportionality of scintillator light yield\ textit {vs} particle energy is a significant issu... more Nonproportionality of scintillator light yield\ textit {vs} particle energy is a significant issue in new-generation radiation analyzers for homeland security, medical, and astrophysical applications. To study kinetics of quenching at high excitation densities characteristic of electron track ends, we have used 0.3 ps pulses of 5.9-eV light to excite up to 5 x 10$^{20} $ eh/cm $^{3} $ in CsI. Streak camera luminescence and transient absorption data show high-density eh pair excitation yielding partially quenched self-trapped exciton (STE) ...
abstract The effect of high excitation density in promoting nonlinear quenching that is 2 nd or 3... more abstract The effect of high excitation density in promoting nonlinear quenching that is 2 nd or 3 rd order in electron-hole density is generally understood to be a root cause of nonproportionality in scintillators. We report and discuss quantitative data on just how fast these nonlinear channels are in specific cases. Kinetic rate constants for the creation of excitons from electrons and holes and for their quenching by dipole-dipole transfer have been measured in CsI and NaI. We show in addition that the strong radial concentration ...
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Papers by Joel Q. Grim