8 research outputs found
Enabling Ambipolar to Heavy nāType Transport in PbS Quantum Dot Solids through Doping with Organic Molecules
PbS quantum dots (QDs) are remarkable semiconducting materials, which
are compatible with low-cost solution-processed electronic device
fabrication. Understanding the doping of these materials is one of
the great research interests, as it is a necessary step to improve
the device performance as well as to enhance the applicability of
this system for diverse optoelectronic applications. Here, we report
the efficient doping of the PbS QD films with the use of solution-processable
organic molecules. By engineering the energy levels of the donor molecules
and the PbS QDs through the use of different cross-linking ligands,
we are able to control the characteristics of PbS field-effect transistors
(FETs) from ambipolar to strongly n-type. Because the doping promotes
trap filling, the charge carrier mobility is improved up to 0.64 cm<sup>2</sup> V<sup>ā1</sup> s<sup>ā1</sup>, which is the
highest mobility reported for low-temperature processed PbS FETs employing
SiO<sub>2</sub> as the gate dielectric. The doping also reduces the
contact resistance of the devices, which can also explain the origin
of the increased mobility
Quasi-epitaxial Metal-Halide Perovskite Ligand Shells on PbS Nanocrystals
Epitaxial growth
techniques enable nearly defect free heterostructures
with coherent interfaces, which are of utmost importance for high
performance electronic devices. While high-vacuum technology-based
growth techniques are state-of-the art, here we pursue a purely solution
processed approach to obtain nanocrystals with eptaxially coherent
and quasi-lattice matched inorganic ligand shells. Octahedral metal-halide
clusters, respectively 0-dimensional perovskites, were employed as
ligands to match the coordination geometry of the PbS cubic rock-salt
lattice. Different clusters (CH<sub>3</sub>NH<sub>3</sub><sup>+</sup>)<sub>(6ā<i>x</i>)</sub>[M<sup>(<i>x</i>+)</sup>Hal<sub>6</sub>]<sup>(6ā<i>x</i>)ā</sup> (M<sup><i>x</i>+</sup> = PbĀ(II), BiĀ(III), MnĀ(II), InĀ(III),
Hal = Cl, I) were attached to the nanocrystal surfaces <i>via</i> a scalable phase transfer procedure. The ligand attachment and coherence
of the formed PbS/ligand core/shell interface was confirmed by combining
the results from transmission electron microscopy, small-angle X-ray
scattering, nuclear magnetic resonance spectroscopy and powder X-ray
diffraction. The lattice mismatch between ligand shell and nanocrystal
core plays a key role in performance. In photoconducting devices the
best performance (detectivity of 2 Ć 10<sup>11</sup> cm Hz <sup>1/2</sup>/W with > 110 kHz bandwidth) was obtained with (CH<sub>3</sub>NH<sub>3</sub>)<sub>3</sub>BiI<sub>6</sub> ligands, providing
the
smallest relative lattice mismatch of <i>ca</i>. ā1%.
PbS nanocrystals with such ligands exhibited in millimeter sized bulk
samples in the form of pressed pellets a relatively high carrier mobility
for nanocrystal solids of ā¼1.3 cm<sup>2</sup>/(V s), a carrier
lifetime of ā¼70 Ī¼s, and a low residual carrier concentration
of 2.6 Ć 10<sup>13</sup> cm<sup>ā3</sup>. Thus, by selection
of ligands with appropriate geometry and bond lengths optimized quasi-epitaxial
ligand shells were formed on nanocrystals, which are beneficial for
applications in optoelectronics
Crystal Phase Transitions in the Shell of PbS/CdS Core/Shell Nanocrystals Influences Photoluminescence Intensity
We reveal the existence of two different
crystalline phases, i.e.,
the metastable <i>rock salt</i> and the equilibrium <i>zinc blende</i> phase within the CdS-shell of PbS/CdS core/shell
nanocrystals formed by cationic exchange. The chemical composition
profile of the core/shell nanocrystals with different dimensions is
determined by means of anomalous small-angle X-ray scattering with
subnanometer resolution and is compared to X-ray diffraction analysis.
We demonstrate that the photoluminescence emission of PbS nanocrystals
can be drastically enhanced by the formation of a CdS shell. Especially,
the ratio of the two crystalline phases in the shell significantly
influences the photoluminescence enhancement. The highest emission
was achieved for chemically pure CdS shells below 1 nm thickness with
a dominant metastable <i>rock salt</i> phase fraction matching
the crystal structure of the PbS core. The metastable phase fraction
decreases with increasing shell thickness and increasing exchange
times. The photoluminescence intensity depicts a constant decrease
with decreasing metastable <i>rock salt</i> phase fraction
but shows an abrupt drop for shells above 1.3 nm thickness. We relate
this effect to two different transition mechanisms for changing from
the metastable <i>rock salt</i> phase to the equilibrium <i>zinc blende</i> phase depending on the shell thickness
From Highly Monodisperse Indium and Indium Tin Colloidal Nanocrystals to Self-Assembled Indium Tin Oxide Nanoelectrodes
Indium tin oxide (ITO) nanopatterned electrodes are prepared from colloidal solutions as a material saving alternative to the industrial vapor phase deposition and top down processing. For that purpose highly monodisperse In<sub>1ā<i>x</i></sub>Sn<sub><i>x</i></sub> (<i>x</i> < 0.1) colloidal nanocrystals (NCs) are synthesized with accurate size and composition control. The outstanding monodispersity of the NCs is evidenced by their self-assembly properties into highly ordered superlattices. Deposition on structured substrates and subsequent treatment in oxygen plasma converts the NC assemblies into transparent electrode patterns with feature sizes down to the diameter of single NCs. The conductivity in these ITO electrodes competes with the best values reported for electrodes from ITO nanoparticle inks
High Infrared Photoconductivity in Films of Arsenic-Sulfide-Encapsulated Lead-Sulfide Nanocrystals
Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are reported. Stable colloidal dispersions of (NH<sub>4</sub>)<sub>3</sub>AsS<sub>3</sub>-capped PbS QDs were processed by a conventional dip-coating technique into a thin homogeneous film of electronically coupled PbS QDs. Upon drying at 130 Ā°C, (NH<sub>4</sub>)<sub>3</sub>AsS<sub>3</sub> capping ligands were converted into a thin layer of As<sub>2</sub>S<sub>3</sub>, acting as an infrared-transparent semiconducting glue. Photodetectors obtained by depositing such films onto glass substrates with interdigitate electrode structures feature extremely high light responsivity and detectivity with values of more than 200 A/W and 1.2 Ć 10<sup>13</sup> Jones, respectively, at infrared wavelengths up to 1400 nm. Importantly, these devices were fabricated and tested under ambient atmosphere. Using a set of time-resolved optoelectronic experiments, the important role played by the carrier trap states, presumably localized on the arsenic-sulfide surface coating, has been elucidated. Foremost, these traps enable a very high photoconductive gain of at least 200. The trap state density as a function of energy has been plotted from the frequency dependence of the photoinduced absorption (PIA), whereas the distribution of lifetimes of these traps was recovered from PIA and photoconductivity (PC) phase spectra. These trap states also have an important impact on carrier dynamics, which led us to propose a kinetic model for trap state filling that consistently describes the experimental photoconductivity transients at various intensities of excitation light. This model also provides realistic values for the photoconductive gain and thus may serve as a useful tool to describe photoconductivity in nanocrystal-based solids
Hydrogen-Bonded Organic Semiconductor Micro- And Nanocrystals: From Colloidal Syntheses to (Opto-)Electronic Devices
Organic pigments such as indigos,
quinacridones, and phthalocyanines
are widely produced industrially as colorants for everyday products
as various as cosmetics and printing inks. Herein we introduce a general
procedure to transform commercially available insoluble microcrystalline
pigment powders into colloidal solutions of variously sized and shaped
semiconductor micro- and nanocrystals. The synthesis is based on the
transformation of the pigments into soluble dyes by introducing transient
protecting groups on the secondary amine moieties, followed by controlled
deprotection in solution. Three deprotection methods are demonstrated:
thermal cleavage, acid-catalyzed deprotection, and amine-induced deprotection.
During these processes, ligands are introduced to afford colloidal
stability and to provide dedicated surface functionality and for size
and shape control. The resulting micro- and nanocrystals exhibit a
wide range of optical absorption and photoluminescence over spectral
regions from the visible to the near-infrared. Due to excellent colloidal
solubility offered by the ligands, the achieved organic nanocrystals
are suitable for solution processing of (opto)Āelectronic devices.
As examples, phthalocyanine nanowire transistors as well as quinacridone
nanocrystal photodetectors, with photoresponsivity values by far outperforming
those of vacuum deposited reference samples, are demonstrated. The
high responsivity is enabled by photoinduced charge transfer between
the nanocrystals and the directly attached electron-accepting vitamin
B2 ligands. The semiconducting nanocrystals described here offer a
cheap, nontoxic, and environmentally friendly alternative to inorganic
nanocrystals as well as a new paradigm for obtaining organic semiconductor
materials from commercial colorants
Size-Dependent Electron Transfer from Colloidal PbS Nanocrystals to Fullerene
We investigate a promising organic/inorganic hybrid composite for solution-processable optoelectronics made by lead sulphide nanoparticles and fullerene derivatives, which combine the sensitivity of PbS to the infrared spectrum with the good electron transport properties of fullerenes. Charge separation is the crucial process that determines whether the heterojunction can be the building block for devices converting photogenerated excitons into free charges flowing in a circuit. Subpicosecond spectroscopy techniques on bulk heterojunctions between PbS nanocrystals of various sizes and [6,6]-phenyl-61-butyric acid methyl ester (PCBM) were employed to reveal the ultrafast dynamics of photoexcited carriers, particularly transfer of photoexcited electrons from nanocrystals to PCBM. Electron transfer is found to critically depend on nanoparticle size, occurring for nanocrystals with diameter 4.4 nm and smaller, not for larger ones. Our findings are relevant to the engineering of hybrid solar cells and light detectors based on PbS nanocrystal/fullerene bulk heterojunctions
Random Lasing with Systematic Threshold Behavior in Films of CdSe/CdS Core/Thick-Shell Colloidal Quantum Dots
While over the past years the syntheses of colloidal quantum dots (CQDs) with core/shell structures were continuously improved to obtain highly efficient emission, it has remained a challenge to use them as active materials in laser devices. Here, we report random lasing at room temperature in films of CdSe/CdS CQDs with different core/shell band alignments and extra thick shells. Even though the lasing process is based on random scattering, we find systematic dependencies of the laser thresholds on morphology and laser spot size. To minimize laser thresholds, optimizing the film-forming properties of the CQDs, proven by small-angle X-ray scattering, was found to be more important than the optical parameters of the CQDs, such as biexciton lifetime and binding energy or fluorescence decay time. Furthermore, the observed systematic behavior turned out to be highly reproducible after storing the samples in air for more than 1 year. These highly reproducible systematic dependencies suggest that random lasing experiments are a valuable tool for testing nanocrystal materials, providing a direct and simple feedback for further development of colloidal gain materials toward lasing in continuous wave operation