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n-Type Conducting CdSe Nanocrystal Solids

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Science  23 May 2003:
Vol. 300, Issue 5623, pp. 1277-1280
DOI: 10.1126/science.1084424

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Abstract

A bottleneck limiting the widespread application of semiconductor nanocrystal solids is their poor conductivity. We report that the conductivity of thin films of n-type CdSe nanocrystals increases by many orders of magnitude as the occupation of the first two electronic shells, 1Se and 1Pe, increases, either by potassium or electrochemical doping. Around half-filling of the 1Se shell, a peak in the conductivity is observed, indicating shell-to-shell transport. Introducing conjugated ligands between nanocrystals increases the conductivities of these states to ∼102 siemens per centimeter.

During the past decade, it has become apparent that solids of monodispersed nanocrystals provide the opportunity for developing materials with novel properties (1, 2). In particular, semiconductor nanocrystals (3) offer great promise for fabricating optoelectrical devices (46). In these “artificial atoms,” the inorganic cores allow precise tuning of the discrete electronic states by size confinement. To stabilize against sintering, retain solubility, and maintain good optical properties, the surfaces are capped by organic ligands, as in the prototypical CdSe system (7). However, these ligands and traps on the nanocrystal surfaces are thought to inhibit electronic transport through solids of the nanocrystals. In one case, CdSe nanocrystal films have extremely low conductivity (∼1014 S cm1 below 200 K) (8) and very poor photoconductivity (9, 10). Photoconductivity studies showed that the mobilities are appreciable (104 to 106 cm2/V·s) only at very high fields of 107 V/m and that a large trap density prevents efficient transport (10). These values contrast with the useful conductivities of nanoporous oxide electrodes, such as ZnO, and TiO2 with mobilities of 1 to 104 cm2/V·s (1114).

Basic studies of the effects of charging on conductivity have had a profound impact on the discovery and design of new conducting materials (15, 16). We report our study of the conductivity of n-type CdSe nanocrystals (17).

Conductivity was readily observed in CdSe nanocrystal thin films made n-type by K evaporation in an ultrahigh vacuum chamber (18). The films are made by drop-casting a hexane-octane solution of nanocrystals on top of gold electrodes (200 μm long, 10-μm gap across electrodes, 80 nm thick) previously patterned on a sapphire substrate. Before K evaporation, the film is insulating below the detection limit (<1 pS), and an infrared (IR) spectrum of the film is taken as a baseline. The average occupation of the quantum states is followed by the IR transmission of the film as K donates its electron to the CdSe nanocrystals. Figure 1 shows typical data for a film of ∼5.4-nm-diameter nanocrystals, with the trioctylphosphine/trioctylphosphine oxide (TOP/TOPO) surface ligands. As evaporation proceeds, electrons first occupy the lowest conduction band quantum state of the nanocrystals denoted 1Se. The IR transition from 1Se to the next state, 1Pe (17), develops around 2100 cm1, and the conductance increases. With further evaporation, the resonance shifts to 2700 cm1, increases in intensity, and exhibits a broader tail in the blue region. These latter effects are attributed to the 1Pe-1De intraband transition, indicating that some of the nanocrystals are multi-charged. Eventually, the conductance and IR intensities saturate. The maximum IR absorption, 0.01 optical density (OD), remains smaller than that of the initial visible exciton (0.04 OD) and corresponds to only ∼six monolayers of singly charged nanocrystals (19). This gives a mobility μ of ∼3 × 106 cm2/V·s at 110 K (18). The conductivity from 100 to 300 K is thermally activated with an activation energy of 57 meV. This energy is similar to values reported for metal nanocrystal arrays of comparable particle size (2022) and is interpreted as the charging energy. These results demonstrate the n-type conductivity of the CdSe nanocrystal solids. However, we could not obtain uniform and complete K-doping through the film necessary for more quantitative conclusions.

Fig. 1.

Simultaneous increase of the electron density and conductivity of a TOP/TOPO-capped CdSe nanocrystal film of ∼5.4-nm diameter and ∼12 monolayer thickness at 110 K in ultrahigh vacuum. The solid line represents the film conductance (1-Vbias, field strength 103 V/cm), and the circles represent the integrated infrared absorbance. The K evaporation begins at t = 0 and continues for 15 min. (Inset) IR spectra at various time points. From bottom to top: 0, 2, 3, 4, 5.5, 7, 8, 9.5, 13, and 15 min. The dotted spectrum was taken 40 min after evaporation was stopped.

With electrochemistry, the carrier density can in principle be more accurately and reversibly tuned. In our experiment, a bipotentiostat controlled the potential of two inter-digitated working electrodes, which were made of 50 pairs of interdigitated Pt strips (5-mm length, 5-μm width, 110-nm thickness, separated by 5 μm covering an area of 0.05 cm2; Abtech Scientific, Richmond, Virginia), as a function of a reference potential provided by a silver wire in the electrolyte solution. For conductivity measurement, a fixed small (<40 mV) potential difference was maintained between the pair of working electrodes. The sum of the currents from the two working electrodes is the total electrochemical current, which arises from the reduction/oxidation of the nanocrystals, impurities, solution resistance, and electrode capacitance as the voltage is scanned. The conduction current is determined as half of the difference between the currents from the two electrodes.

Using TOP/TOPO-capped CdSe nanocrystals (23), we failed to detect conduction across the film. However, by treating the films with cross-linking molecules, which leads to much faster and reversible voltammetry (24), we were able to fully charge nanocrystals across the electrode gap. Figure 2 shows the voltammetric and conductivity measurements on such cross-linked films. For TOP/TOPO-capped CdSe nanocrystals that have been cross-linked with 1,7-heptanediamine, the conductance shows two steps that coincide with the two waves in the reduction current. From the conductance G and the integrated charge density Q, the mobility is extracted (25). The differential mobility (Fig. 2B) starts in the noise near 0 V, but stabilizes at ∼0.8 × 105 cm2/V·s before the first reduction current peak. It then decreases sharply but increases again to ∼0.8 × 104 cm2/V·s before the peak of the second reduction wave before decreasing again. The anodic scan is similar.

Fig. 2.

Cyclic voltammetry of a TOP/TOPO-capped CdSe nanocrystal film of ∼6.4-nm diameter and ∼60-nm thickness treated with 1,7-heptanediamine. (A) Cyclic voltammetry at 33 mV/s. The bias was 8 mV. The arrows indicate the cycle direction. The dotted line (linear scale) is the electrochemical current, and the solid line (logarithmic scale) is the conduction current. (B) Integrated surface charge density of the cathodic half-cycle (dotted line, linear scale) and differential mobility (solid line, logarithmic scale). The electrolyte is 0.1 M tetrabutylammonium perchlorate in N,N-dimethylformamide, cooled to –60°C. The electrochemical current shows a nearly linear dependence on scan rate, whereas the conductance between the electrodes mostly exhibits increasing hysteresis and overpotential with faster scan rates.

The two differential mobility maxima are assigned to the opening of the 1Se and 1Pe shells of the quantum dots. The well-characterized spectroscopy of CdSe nanocrystals (26) provides unambiguous evidence for the occupation of these electronic states (fig. S2). At a slower scan rate for the same sample with simultaneous optical measurements (Fig. 3A), the first rise in conductance follows linearly the occupation of the 1Se shell up to about half-filling, after which it plateaus and even exhibits a small decrease. The conductance increases again by a factor of 30 as the 1Pe shell opens. The ∼10-fold larger mobility of the 1Pe electrons is likely due to the increased wave-function overlap of the more energetic Pe states. The further rise of the conductivity below –0.8 V may be caused by the opening of the 1De or 2Se shell, but we have no independent confirmation from the optical spectra. For the ∼5.4-nm-diameter nanocrystal sample (Fig. 3B), the conductance peak is very marked and occurs at ∼60% filling of the 1Se shell. However, the optical data also show that the 1Pe shell cannot be completely filled, presumably because the P electrons oxidize more readily.

Fig. 3.

Conductance and optical bleach data for two samples at –60°C. The scan rate is 0.5 mV/s. (A) CdSe nanocrystals of ∼6.4-nm diameter, as in Fig. 2. The solid line represents the conductance (bias 35 mV). The open circles represent the bleach magnitude (OD) at the 1S exciton (620 nm), and the open squares the bleach magnitude at the 1P exciton (548 nm). (B) CdSe nanocrystals of ∼5.4-nm diameter. The solid line represents the conductance (bias 35 mV). The open circles represent the bleach magnitude at the 1S exciton peak (593 nm), and the open squares the bleach magnitude at the 1P exciton (520 nm). In both films, the first exciton is completely bleached at the most negative potentials. The potentials are measured against the Ag pseudo-reference electrode.

In a one-shell system, the peak at half-filling is expected because electron transfer from occupied state to empty state should scale as x(1–x), where x is the filling fraction. Previous work on ZnO nanocrystal films (13) showed only a monotonous increase of the conductance with charging. In CdSe, the different shells are energetically well separated, charging is independently ascertained by optical data, and energetic disorder arising, for example, from the size dispersion is sufficiently small that the conductance peaks are visible.

The electrochemical cell can be cooled at negative potential until the electrolyte solution freezes (∼ –73°C), after which the bipotentiostat can be disconnected. The samples then remain conducting indefinitely. Upon warm-up, the conductivity increases following an activated behavior. Between 200 and 80 K, an activation energy of 36 meV was measured for the ∼6.4-nm-diameter CdSe nanocrystals (fig. S1). The n-type CdSe nanocrystal solids exhibit ohmic behavior down to 4 K for small bias. Below 30 K and at much higher bias (∼5 V), the films show highly nonlinear current-voltage characteristics. These are similar to those attributed to the Coulomb blockade in gold nanocrystal films (20).

For practical application, higher conductivity than achieved here will be desirable. Shorter ligands with low electronic barriers should improve the conductivity. Replacing the TOP/TOPO capping molecules with pyridine while retaining the 1,7-heptanediamine linker improved the conductivity 10-fold (Fig. 4). A 1000-fold improvement was obtained with a 1,4-phenylenediamine linker. These films have the same optical spectra and can be fully charged in the 1Se shell. Remarkably, they cannot be charged in the 1Pe shell, probably because the 1,4-phenylenediamine introduces trap states above the 1Se shell. This argument is supported by a small, irreversible reduction current seen above the 1Se filling. At the 1Se filling, the mobility μ is ∼0.8 × 102 cm2/V·s, and the conductance σ is ∼0.6 × 102 S cm1. The latter value corresponds to an effective resistance R between nanocrystals of ∼200 megohm [R ∼ (aσ)1, where a is the interparticle distance] and a hopping time τ between nanocrystals of ∼0.5 ns as determined by the diffusion relation, μ = ea2/6kBTτ (where e is the electron charge, kB is the Boltzmann constant, and T is the temperature). It should be possible to achieve an even lower resistance and a faster transfer rate.

Fig. 4.

(A) Conductivity of ∼6.4-nm-diameter CdSe nanocrystals as a function of the potential for different ligands. Dotted line and triangles: TOP/TOPO-capped nanocrystals, film cross-linked with 1,7-heptanediamine (curve 1) (–60°C, 35 mV bias). Dashed line and open circles: pyridine-capped nanocrytals, film cross-linked with 1,7-heptanediamine (curve 2) (–60°C and 35-mVbias). Solid line and filled circles: pyridine-capped nanocrystals, film cross-linked with 1,4-phenylenediamine (curve 3) (22°C, 40 mVbias). Intervals of 30 data points are marked by a symbol. The potential is measured against the Ag pseudo-reference electrode. (B) Same conductivity data as in (A) plotted on a linear scale with respect to the 1S exciton bleach normalized to its maximum value. Open triangles (curve 1), ×1000; open circles (curve 2), ×100; filled circles (curve 3), ×1.

CdSe nanocrystal solids have previously been considered very poor conductors. Here we have demonstrated a conductivity that is 12 orders of magnitude greater than previously reported, up to ∼102 S cm1, comparable to that of nanocrystalline oxide networks and of some conducting polymers. These findings open up tremendous possibilities for exploiting the highly designable visible and infrared optical properties of quantum dots of various shapes and compositions, along with the useful conductivity.

Supporting Online Material

www.sciencemag.org/cgi/content/full/300/5623/1277/DC1

Materials and Methods

Figs. S1 and S2

References and Notes

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