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PbSe Nanocrystal Solids for n- and p-Channel Thin Film Field-Effect Transistors

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Science  07 Oct 2005:
Vol. 310, Issue 5745, pp. 86-89
DOI: 10.1126/science.1116703

Abstract

Initially poorly conducting PbSe nanocrystal solids (quantum dot arrays or superlattices) can be chemically “activated” to fabricate n- and p-channel field effect transistors with electron and hole mobilities of 0.9 and 0.2 square centimeters per volt-second, respectively; with current modulations of about 103 to 104; and with current density approaching 3 × 104 amperes per square centimeter. Chemical treatments engineer the interparticle spacing, electronic coupling, and doping while passivating electronic traps. These nanocrystal field-effect transistors allow reversible switching between n- and p-transport, providing options for complementary metal oxide semiconductor circuits and enabling a range of low-cost, large-area electronic, optoelectronic, thermoelectric, and sensing applications.

Solution-based processes such as spin coating, dip coating, and inkjet printing offer substantial cost reductions for the fabrication of electronic and optoelectronic devices when combined with materials such as organic semiconductors (1), carbon nanotubes (2), nanowires (3), soluble precursors for inorganic semiconductors (4), and hybrid organic-inorganic films (5). However, none of these approaches can yet enable devices with performance comparable to that of conventional inorganic crystalline semiconductors. Improvement of the electronic performance in these systems comes at the price of high-precision and low-throughput fabrication techniques (3) or else requires high-temperature anneals that limit compatibility with flexible plastic substrates. The trade-off between device performance and fabrication costs motivates the search for new classes of materials for low-cost electronics. Here, we report the assembly of solid-state field-effect transistors (FETs) from solution-processable semiconductor nanocrystals.

Charge transport in an array of nanocrystals separated by insulating capping ligands (a “nanocrystal solid”) depends on matching of the energy levels of neighboring nanocrystals (site energies, α), on the exchange coupling energy between the nanocrystals (β), and on the Coulomb charging energy of the nanocrystal array (Ec) (6, 7). For efficient charge transport, the dispersion of site energies Δα should not exceed β, or Anderson localization will dominate. If β < Ec, the nanocrystal array can behave as a Mott insulator (8).

Past studies have revealed low electronic conductivity in semiconductor nanocrystal arrays because of poor exchange coupling and large concentrations of surface dangling bonds that trap carriers in mid-gap states (912). Sintering individual nanocrystals into a polycrystalline film increases film conductance but leaves structural defects that limit the device switching speeds (13). Another approach to enhance electron mobility in a nanocrystal solid is based on cross-linking of the nanocrystals by conjugated organic molecules (e.g., 1,4-phenylenediamine) followed by electrochemical charging with several additional electrons per nanocrystal (1416). Charge screening by the electrolyte's mobile ions substantially reduces Ec, facilitating the nanocrystal charging (12). The mobility increases because of a combination of trap filling and the participation of multiple quantum confined electronic states (1S, 1P, etc.) in charge transport (12, 14). However, technological implementations require FETs with insulated gates capacitively coupled to the transistor channel.

We selected PbSe nanocrystals because they allow smaller Δα and Ec and larger β values, compared to the extensively studied CdSe and ZnO nanocrystal solids. In an ensemble of strongly confined semiconductor nanocrystals, charge transport occurs between electronic quantum confined orbitals (12), and Δα can be estimated from the linewidth of the first excitonic (1Sh-1Se) transition (15). We optimized the synthesis (16) to obtain monodisperse (<5% SD) PbSe nanocrystals, with the full width at half maximum of the 1Sh-1Se transition below 40 meV (Fig. 1, A and B). The 1S quantum confined orbitals in rock-salt PbSe nanocrystals are eight-fold degenerated (17) versus the two-fold spin degeneracy in II-VI nanocrystals. Higher degeneracy and narrower linewidth provide a higher density of electronic states (DOS) available for the charge transport. In a close-packed array of 8-nm PbSe nanocrystals, the densities of 1Sh and 1Se states are each ∼5 × 1020 eV–1 cm–3, an order of magnitude higher than the DOS in arrays of CdSe or ZnO nanocrystals of the same size and ∼330 times the DOS in amorphous germanium (15).

Fig. 1.

(A) Optical absorption spectrum of a colloidal solution of 8-nm PbSe nanocrystals in tetrachloroethylene. a.u., arbitrary units; fwhm, full width at half-maximum. (B) TEM image of an array of 8-nm PbSe nanocrystals. qy and qz are the lateral and vertical components of the momentum transfer, respectively. (C) GISAXS pattern of PbSe nanocrystal film (16).

The exchange coupling energy scales approximately as β ∼ exp[–κ(d + δ)], where d is the nanocrystal diameter, δ is the interparticle spacing, and κ–1 describes the length scale of the wave function leakage outside the nanocrystal (6, 7). The large Bohr radius of electrons and holes in PbSe (∼23 nm in both) suggests that their wave functions spill far outside the volume of the nanocrystal, facilitating exchange interactions.

The charging energy Ec of a spherical nanocrystal can be estimated as Ec = e2/(4πϵmϵ0d) where ϵm is the dielectric constant of the surrounding medium. The dialectric constant for bulk PbSe is very high (ϵ ∼ 250, as compared to ϵ ∼ 6.2 for CdSe), with ϵ > 100 measured for individual 12-nm PbSe nanocrystals (18). This predicts an Ec of <4 meV for a three-dimensional array of 8-nm PbSe nanocrystals, which is a small fraction of thermal energy at room temperature (kBT, where kB is the Boltzmann constant and T is the temperature) and an order of magnitude smaller than Ec for analogous CdSe nanocrystal arrays (16).

For electronic studies, close-packed PbSe nanocrystal films, 35 ± 10 nm thick, were deposited on highly doped Si wafers with 100-nm-thick SiO2 thermal gate oxide. Source and drain Ti/Au (75/375 Å) electrodes spaced from 4 μm to 40 μm apart were patterned on the SiO2 surface before nanocrystal deposition (16).

Small angle x-ray scattering at grazing incidence (GISAXS) from 8-nm PbSe nanocrystals assembled in the channel between parallel electrodes showed well-resolved reflections (Fig. 1C), confirming both in-plane and vertical particle ordering in a film that was drop-cast from a hexane:octane (9:1 by volume) solution. The parallel source and drain electrodes directed the crystallographic orientation of growing superlattices (fig. S1). Moreover, close-packed PbSe nanocrystals showed strong preferential orientation of their atomic lattices, favorable for charge transport through P-quantized states (figs. S2 and S3)

As-deposited PbSe nanocrystal arrays were insulating, with a conductance (G) less than 10–11 S cm–1, because of ∼1.5-nm interparticle spacing maintained by insulating oleic acid molecules (Fig. 1B). Thorough washing of the nanocrystal colloids (16) removed a fraction of the native capping groups, reducing interparticle spacing to ∼1.1 nm and yielding a higher conductance of G ∼ 3 × 10–10 Scm–1. However, the removal of capping groups introduced pronounced hysteresis in the current-voltage (I-V) scans (Fig. 2A), probably due to filling of traps associated with surface dangling bonds. No gate modulation was observed in the native nanocrystal films.

Fig. 2.

(A) I-V scans for a film of 8-nm oleic acid capped PbSe nanocrystals in which the voltage scan rate is 0.5 V s–1 (black) and 0.025 V s–1 (red) and the channel length L and width W are 6 μm and 5000 μm, respectively. (B) Conductance G of a PbSe nanocrystal film versus time of exposure to a 1 M solution of N2H4 in acetonitrile. The error bars show the spread in data from five samples. (C) Plot of drain current ID versus drain-source voltage VDS, as a function of VG for a nanocrystal FET with a channel composed of 8-nm PbSe nanocrystals treated with hydrazine solution for 12 hours (L = 10 μm, W = 2000 μm, with a 100-nm-thick SiO2 gate dielectric).

The conductance of PbSe nanocrystal solids increased by ∼10 orders of magnitude after treatment with a 1.0 M solution of hydrazine in acetonitrile (Fig. 2B) (16). The current through the nanocrystal film can be modulated by application of a potential to the back gate electrode, producing an n-FET (1) (Fig. 2C). Figure 3A shows a current modulation Ion/Ioff of ∼2.5 × 103 for a PbSe nanocrystal n-FET, with minor hysteresis between gate voltage (VG) scans in the forward and reverse directions (fig. S4). In the “on”state, low-field conductance of PbSe nanocrystal film was ∼0.82 S cm–2 (VG = 40 V), and the current density in the saturation regime approached 2.7 × 104 A cm–2. Extracting field-effect electron mobilities (μ) from a series of devices yielded μlin ∼ 0.4 cm2 V–1 s–1 in the linear regime and μsat ∼ 0.7 cm2 V–1 s–1 in the saturation regime (16). The μsat values increased with nanocrystal size. The highest mobility, μsat = 0.95 cm2 V–1 s–1, was observed for 9.2-nm PbSe nanocrystals.

Fig. 3.

Device characteristics of PbSe nanocrystal FETs activated with hydrazine. (A) Plots of ID (solid line) and I D 1/2 (open circles) versus VG at a constant VDS = 40 V, used to calculate current modulation and field-effect mobility in the saturation regime for an n-channel FET assembled from 8.1-nm PbSe nanocrystals (L = 8 μm, W = 2300 μm). (B) ID versus VG plot at a constant VDS = 1 V for an ambipolar FET assembled from 8.1-nm PbSe nanocrystals (L = 8 μm, W = 2300 μm). (C) Plots of ID and I D 1/2 versus VG at a constant VDS = –40 V for a p-channel FET assembled from 8.2-nm PbSe nanocrystals (L = 10 μm, W = 3000 μm). (D) Plot of ID versus VDS, as a function of VG for a p-channel FET assembled from 8.4-nm PbSe nanocrystal (L = 8 μm, W = 2300 μm). The data shown in (A), (B), and (D) were measured at room temperature, whereas the data in (C) were measured at 120 K. The thickness of the SiO2 gate dielectric was 100 nm.

Vacuum treatment or mild heating (to ∼100°C) of activated PbSe nanocrystal films switched their conductivity from n-type (Fig. 3A) to ambipolar (Fig. 3B) and, finally, to p-type (Fig. 3, C and D) as the hydrazine desorbed. The resulting p-FETs showed room-temperature hole mobilities μsat of 0.12 to 0.18 cm2 V–1 s–1, current modulations of ∼102 (Fig. 3D), and “on”state current densities approaching ∼3 × 103 A cm–2. At 120 K, current modulation increased to ∼1.6 × 104, whereas the hole mobility decreased to μsat = 0.09 cm2 V–1 s–1 and was almost independent of the gate voltage (Fig. 3C). The hole transport in PbSe nanocrystal solids most probably occurs through the 1Sh orbitals (19). Switching between electron and hole transport was reversible upon re-exposure to hydrazine, allowing fabrication of complementary metal oxide semiconductor circuits.

Scanning and transmission electron microscopy (SEM and TEM) and x-ray diffraction studies showed that the hydrazine treatment did not change nanocrystal size or shape but markedly reduced the interparticle spacing (Fig. 4A and fig. S3). The reflections in GISAXS patterns shifted to higher scattering (2θ) angles, showing that interparticle spacing decreased by 0.8 nm, i.e., from ∼1.1 nm to ∼0.3 nm (Fig. 4B) (16). Once hydrazine-treated, films neither dissolved nor swelled on re-exposure to nonpolar solvents. This allowed us to pattern PbSe nanocrystal films. Local exposure to hydrazine solutions rendered the regions conductive, whereas unexposed film could be lifted off in hexane. Sequential layer-by-layer deposition would allow different material combinations, e.g., p- and n-conducting layers for designing nanocrystal-based photovoltaic cells.

Fig. 4.

(A) High-resolution SEM image of a PbSe nanocrystal film treated with a 1 M N2H4 solution for 12 hours. (B) Small-angle x-ray scattering from an array of 8.1-nm PbSe nanocrystals before (red) and after (blue) the hydrazine treatment. 2θ, scattering angle. (C) Evolution of the absorption spectrum of the 7.5-nm PbSe nanocrystal film with time during a 4-hour hydrazine treatment (16). The initial absorption spectrum is shown in red. The infrared absorption bands from the solvent are marked by blue asterisks, and the dashed line indicates the position of the 1Sh-1Se excitonic transition in the initial, insulating nanocrystal film.

The absorption spectra of the conductive PbSe nanocrystal films show excitonic peaks that shifted by ∼20 meV to lower energy relative to native insulating films (Fig. 4C). The persistence of excitonic peaks implies the electron and hole wave functions either remain localized on the individual PbSe nanocrystals or form narrow minibands due to exchange coupling (20). At low temperatures, charge transport in PbSe nanocrystal solids occurred through variable range hopping, as confirmed by linearization of low-field conductance in the Mott coordinates (lnGT –¼) (8) (fig. S5).

The hydrazine treatment can simultaneously tune β, Ec, trap density, and the doping level of a PbSe nanocrystal solid. Hydrazine is a Brönsted base and can gently react to remove and replace the bulky oleic acid capping ligands from the nanocrystal surface, reducing the interparticle spacing and increasing β. Hydrazine is also a strong Lewis base with lone pairs of electrons that can saturate dangling bonds at the nanocrystal surface in analogy to primary amines (21). We might also speculate about linking PbSe nanocrystals by bidentate hydrazine molecules, as the mean interparticle spacing is close to the length of a hydrazine molecule. Hydrazine is a reducing agent, preventing oxidation of PbSe nanocrystals and “repairing” any oxidized selenium surface sites that might generate mid-gap levels (22). Replacement of oleic acid (ϵ ∼ 2) with hydrazine (ϵ ∼ 52) also substantially reduces Ec. The increase of β and decrease of Ec helps to close the Hubbard gap (8), enabling the insulatormetal Mott transition in the nanocrystal solid. Finally, hydrazine behaves as a charge-transfer n-type dopant, as has been observed for PbSe nanowires (23) and carbon nanotubes (24).

Annealing the activated PbSe nanocrystal films at ∼200°C for 1 hour and re-exposing them to a hydrazine solution allowed us to achieve degenerate doping and metallic conductivity (G ∼ 8.5 S cm–2) (fig. S6). The semiconductor-metal transition was reversible, as partial stripping of the hydrazine (6 hours at 40°C under nitrogen) yielded n-type semiconducting films with high electron mobility (μlin ∼ 2.5 cm2 V–1 s–1) (fig. S7), thus demonstrating that tailoring of β and the doping density allows controllable switching between insulating, semiconducting, and metallic states in PbSe nanocrystal solids.

Colloidal nanocrystals can now enable room-temperature fabrication of n- and p-channel field effect devices by inexpensive and high-throughput solution-based processes. We observed good performance for PbSe nanocrystal FETs, even for long (e.g., 40 μm) channels (fig. S8). Such device dimensions are easily accessible by stamping or inkjet-printing. The observed field-effect mobilities in PbSe nanocrystal films are comparable to the hole mobility in pentacene films (2), although lower than the electron mobility in the best solution-processed inorganic semiconductors (4).

The hydrazine treatment is a general technique for increasing conductance in nanocrystal solids. In addition to PbSe FETs, we have assembled operational solid-state FETs from PbS, PbTe, CdSe, and InP nanocrystals and CdSe nanorods, thus demonstrating the applicability of our approach to different materials.

Supporting Online Material

www.sciencemag.org/cgi/content/full/310/5745/86/DC1

Materials and Methods

Figs. S1 to S8

References and Notes

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