Solution-processed electronic and optoelectronic devices offer low cost, large device area, physical flexibility and convenient materials integration compared to conventional epitaxially grown, lattice-matched, crystalline semiconductor devices. Although the electronic or optoelectronic performance of these solution-processed devices is typically inferior to that of those fabricated by conventional routes, this can be tolerated for some applications in view of the other benefits. Here we report the fabrication of solution-processed infrared photodetectors that are superior in their normalized detectivity (D*, the figure of merit for detector sensitivity) to the best epitaxially grown devices operating at room temperature. We produced the devices in a single solution-processing step, overcoating a prefabricated planar electrode array with an unpatterned layer of PbS colloidal quantum dot nanocrystals. The devices showed large photoconductive gains with responsivities greater than 10(3) A W(-1). The best devices exhibited a normalized detectivity D* of 1.8 x 10(13) jones (1 jones = 1 cm Hz(1/2) W(-1)) at 1.3 microm at room temperature: today's highest performance infrared photodetectors are photovoltaic devices made from epitaxially grown InGaAs that exhibit peak D* in the 10(12) jones range at room temperature, whereas the previous record for D* from a photoconductive detector lies at 10(11) jones. The tailored selection of absorption onset energy through the quantum size effect, combined with deliberate engineering of the sequence of nanoparticle fusing and surface trap functionalization, underlie the superior performance achieved in this readily fabricated family of devices.

Ultrasensitive solution-cast quantum dot photodetectors

The creation of perfect nanometer-scale crystallites (nanocrystals), identically replicated in unlimited quantities, in a state that can be manipulated and understood as pure macromolecular substances, is an ultimate challenge of modern materials research with outstanding fundamental and potential technological consequences.[’l We report on the prediction, isolation, and characterization of a series of gold nanocrystals, passivated by self-assembled monolayers (SAMs) of straight-chain alkylthiolate molecules (RS, R

Nanocrystal gold molecules

Within the wr-orbital axis vector theory, the total rehybridization required for closure of the fullerenes is approximately conserved. This result allows the development of a structure-based index of strain in the fullerenes, and it is estimated that about 80 percent of the heat of formation of the carbon atoms in C60 may be attributed to a combination of v strain and steric inhibition of resonance. Application of this analysis to the geometries of structurally characterized organometallic derivatives of C60 and C70 shows that the reactivity exhibited by the fullerenes may be attributed to the relief of a combination of local and global strain energy. C60 is of ambiguous aromatic character with anomalous magnetic properties but with the reactivity of a continuous aromatic molecule, moderated only by the tremendous strain inherent in the spheroidal structure.

https://science.sciencemag.org/content/sci/261/5128/1545.full.pdf

Chemistry of the fullerenes: the manifestation of strain in a class of continuous aromatic molecules.

Carbazole-containing polymers: synthesis, properties and applications

A series of giant metal-cluster compounds, each composed of a gold core and a glutathione (GSH) adsorbate layer, have been prepared from Au(I)SG polymers and separated by gel electrophoresis, using methods reported in a recent Letter [J. Phys. Chem. B 1998, 102, 10643−6]. Identification of the separated compounds by core mass is accomplished through laser desorption mass spectrometry of matrix-diluted films. Three principal compounds have core masses of ca. 4.3, 5.6, and 8.2 kDa (in the range of ∼20−40 Au atoms), and show structured optical absorption spectra with clear optical absorption onsets near 1.7, 1.3, and 1.0 eV, respectively. Each of these shows unusually strong chiroptical activity in the metal-based electronic transitions across the near-infrared, visible, and near-ultraviolet regions, whereas neither the crude (unseparated) mixture nor its higher molecular weight components possess such strong optical activity. The location and strength of the optical activity suggest a metal electronic struc...

Giant Gold−Glutathione Cluster Compounds: Intense Optical Activity in Metal-Based Transitions

The toluene extract of the fluffy carbon material produced by resistive heating of graphite contains a variety of molecules larger than C(60) and C(70) in a total amount of 3 to 4% by weight. Repeated chromatography of this material on neutral alumina has led to the isolation of stable solid samples of C(76), C(84), C(90), and C(94). The characterization, which includes mass spectrometry, (13)C nuclear magnetic resonance, electronic absorption (ultraviolet/visible) and vibrational (infrared) spectroscopy identifies these all-carbon molecules as higher fullerenes. In addition, C(70)O, a stable oxide, has been isolated that is structurally and electronically closely related to D5h-C(70). This compound forms during the resistive heating process and probably has an oxygen atom inserted between two carbon atoms on the convex external surface of the C(70) skeleton.

https://science.sciencemag.org/content/sci/252/5005/548.full.pdf

The Higher Fullerenes: Isolation and Characterization of C76, C84, C90, C94, and C70O, an Oxide of D5h-C70

A LONG search for molecular allotropic forms of carbon1–4 culminated in the discovery5–7 of a method for preparing large quantities of the C60 molecule and the subsequent confirmation8,9 of its cage-like truncated-icosahedral structure1. The C70 molecule prepared by the same method was later also isolated and found to have the predicted cylindrical (D5h) structure. Incomplete chromatographic separation of the large molecules C76, C78 and C84 was achieved at the same time10,11, and small quantities of highly enriched samples were later isolated12. Preliminary NMR and spectroscopic studies of these carbon clusters failed, however, to provide evidence for the symmetrical structures predicted earlier13. Here we report the isolation and characterization of the C76 molecule, a third molecular form of carbon following C60 and C70. We isolated and purified substantial quantities of this species using the extraction technique of ref. 6 together with chromatography. The 13C NMR spectrum consists of 19 lines of essentially equal intensity, confirming a compact, cage-like fullerene structure. Among the several thousand possible structures for C76, theoretical calculations by Manolopolous14 predict a chiral structure with D2 symmetry, consisting of a spiralling, double-helical arrangement of edge-sharing pentagons and hexagons. Our NMR spectrum is uniquely consistent with this remarkable structure.

Isolation of C76, a chiral (D2) allotrope of carbon

Early reports on the formation of the higher fullerenes C 76 , C 78 , C 84 , C 90 , and C 94 by resistive heating of graphite stimulated theoretical calculations of possible cage structures for these all-carbon molecules. Among the five fullerene structures with isolated pentagons found for C 78 , a closed-shell D 3 h -isomer was predicted to form preferentially. Two distinct C 78 -isomers were formed in a ratio of ∼5:1 and could be separated by high-performance liquid chromatography. The carbon-13 nuclear magnetic resonance (NMR) spectrum of the major isomer is uniquely consistent with a C 2 v -structure. The NMR data also support a chiral D 3 -structure for the minor isomer. The isolation of specifically these two isomers of C 78 provides insight into the stability of higher fullerene structures and into the mechanism for fullerene formation in general.

https://science.sciencemag.org/content/sci/254/5039/1768.full.pdf

Fullerene isomerism: isolation of c2v,-c78 and d3-c78.

A report is given on molecular carbon research in Los Angeles, starting with review of the molecular precursor approach to carbon materials.1 Regarding larger molecules, mixtures of carbon molecules, Cn, are extracted2 in 25‐40 percent yield from graphite evaporated under helium. Gram quantities of the mixture are then separated into pure fractions — C60, C70, C76/C78, C84, and higher (n > 88),3,4 and characterized by mass spectrometry, liquid chromatography, and 13CNMR spectroscopy. Examination of the spectroscopic and photophysical5 properties of the molecules, with particular focus on C60 in solution, reveals: (i) the first transitions appear at high energy, are electronically forbidden (in C60, the homo-lumo hu - t1u transitions), and exhibit resolved vibronic structure; (ii) the spectra sharpen in cryogenic media, allowing vibronic assignment; and (iii) relaxation occurs with near-unity quantum yield to a long-lived triplet, which also transfers energy efficiently to acceptors including O2.

Roland Ettl

Papers

The Higher Fullerenes: Isolation and Characterization of C76, C84, C90, C94, and C70O, an Oxide of D5h-C70

Isolation of C76, a chiral (D2) allotrope of carbon

Fullerene isomerism: isolation of c2v,-c78 and d3-c78.

Spectroscopic and Photophysical Properties of the Soluble C n Molecules, n = 60, 70, 76/78, 84

Isolation of C76, a Chiral (D2) Allotrope of Carbon.