Nanocrystals: Size-Dependent Properties and Emerging Applications Part 1 (Nanotechnology)

INTRODUCTION

Nanocrystals have long been known for their ability to color glasses and catalytic action. New understanding of nanoscale phenomena, especially the changes in the material properties with size are promising to usher in a whole new genre of applications.^1-3 In this entry, a discussion of the electronic, optical, and magnetic properties of capped nanocrystals, followed by a brief elucidation of select applications is provided.

ELECTRONIC AND RELATED PROPERTIES

As the size of nanocrystals is varied in the nanometric domain, the electronic structure undergoes various changes. The changes were theoretically addressed as early as the 1960s by Kubo and Frolich.[4] Kubo successfully predicted that a gap, now called the Kubo gap (A), will emerge as the dimensions are reduced:

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where, Ef is the bulk Fermi level and n the number of free electrons in the nanoparticle (a contribution of one electron per atom is usually assumed). A natural consequence of the emergence of Kubo gap is a size-induced metal-insulator transition when the diameter of the particle is decreased to below a few nano-meters.[5-7] Scanning tunneling spectroscopy (STS) measurements of nanocrystals of various metals have revealed that the nanocrystals of dimensions ~1nm3 exhibit a definitive band gap (up to 70meV) that decreases gradually as the volume of the nanocrystal increases (see Fig. 1).[6,8,9] Photoelectron spectroscopic measurements on mass selected Hgn nanoparticles (n = 3-250) in the gas phase reveal that the characteristic HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) (s-p) energy gap decreases gradually from ~3.5 eV for n < 3 to ^0.2 eV for n < 250, as shown in Fig. 2.


The band gap closure is predicted at n ~ 400. In addition to A, nanoparticles also possess coulombic energy states that manifest themselves when actual charging and discharging events take place. Experiments have revealed that small nanocrystals possess charging energies (U) of the order of hundreds of millielectron Volts. Therefore, in ensembles of nano-crystals, charge transport would be dominated by a hopping mechanism. A large number of studies have addressed charge-transport characteristics of nanocrystalline assemblies. Pellets of monodisperse nanocrystals, obtained by the use of bifunctional ligand that may bind to more than one nanocrystal, or by applying pressure on dried nanocrystalline matter, have been used for electrical transport measurements.[10-13] Pellets made of small Au and Pd nanocrystals exhibit nonmetallic behavior with specific conductivities in the range of 106 Q-1 cm . ] The conductivity, however, increases dramatically with an increase in the diameter of the nanocrystals. An insulator metal transition has indeed been reported from pellets made of ~12.5 nm Au and Ag nanocrystals.[13] Electrical transport measurements on layer-by-layer assemblies ofnanocrys-tals on conducting substrates have been carried out by adoption of a sandwich configuration.[14-16]

Nanocrystalline films with bulk metallic conductivity have been realized with Au nanocrystals of 5 and 11 nm diameter spaced with ionic and covalent spacers.[15,16] The conductivity of monolayered two-dimensional arrays of metal nanocrystals has been studied with patterned electrodes.[17-22] Structural disorder and inter-particle separation distance are identified as key factors that determine the conductivity of such layers.[17-20] The conductivity of such layers can be enhanced by replacing alkane thiol with an aromatic thiol insitu.[21,22] That the interaction energy of nanocrystals in such organizations can be continually varied by changing the interparticle distance was exploited by Heath and coworker who prepared a monolayer of Ag (~3 nm) nanocrystals at air/water interface in a Langmuir Blodgett trough and varied the interparticle distance by applying pressure.[23,24] A host of measurements including reflectivity and nonlinear optical spectroscopic techniques were carried out insitu. This study led to the observation of a reversible Mott-Hubbard metal-insulator transition in the nanocrystal ensemble wherein the coulomb gap closes at a critical distance between the particles. Tunneling spectroscopic measurements on films of 2.6 nm Ag nanocrystals capped with decanethiol reveal a coulomb blockade behavior attributable to isolated nanocrystals.[24] On the other hand, nanocrystals capped with hexane and pentane thiol exhibit characteristics of strong interparticle quantum mechanical exchange (see Fig. 3). Similar behavior was observed in the case of self-assembled two-dimensional arrays of Co nanocrystals and Au nanocrystals.[25,26] By varying simple parameters in the synthetic scheme, Rao and coworkers have been able to tune the specific resistivity of a thin film of Au nanocrystals in the range of tens of megaohms to a few ohms (see Fig. 4). Along with the change in conductivity, the nature of the film also undergoes a change from that of an insulator to a metal. Schmid and coworkers as well as Pal et al. have obtained rectifying behavior from ensembles of nano-crystals (see Fig. 5).[27,28] Although a clear understanding is yet to emerge, it is clear that the observed rectification is related to the high charging energies of the individual nanocrystals.

Variation of the nonmetallic band gap with nanocrystal size in metal nanocrystals. The bandgaps were obtained based on STS measurements.

Fig. 1 Variation of the nonmetallic band gap with nanocrystal size in metal nanocrystals. The bandgaps were obtained based on STS measurements.

Photoelectron spectra of Hg clusters of varying nuclearity. The 6p feature moves gradually toward the Fermi level, emphasizing that the band gap shrinks with increase in cluster size.

Fig. 2 Photoelectron spectra of Hg clusters of varying nuclearity. The 6p feature moves gradually toward the Fermi level, emphasizing that the band gap shrinks with increase in cluster size.

Normalized density of states (DOS) measured from arrays of Ag nanocrystals of diameter ~2.6nm capped (A) decanethiol and (B) hexanethiol at various temperatures. The temperature dependence of DOS near 0 V for decanethiol capped particles indicates that the films are nonmetallic. In the case of hexanethiol capped nanocrystals, the DOS around 0 V is temperature independent revealing the metallic nature of the film.

Fig. 3 Normalized density of states (DOS) measured from arrays of Ag nanocrystals of diameter ~2.6nm capped (A) decanethiol and (B) hexanethiol at various temperatures. The temperature dependence of DOS near 0 V for decanethiol capped particles indicates that the films are nonmetallic. In the case of hexanethiol capped nanocrystals, the DOS around 0 V is temperature independent revealing the metallic nature of the film.

The resistance of Au nanocrystal films as a function of temperature. The films (A)-(D) were obtained by carrying out the synthesis at different temperatures: (A) 303 K; (B) 318 K; (C) 333 K; and (D) 348 K.

Fig. 4 The resistance of Au nanocrystal films as a function of temperature. The films (A)-(D) were obtained by carrying out the synthesis at different temperatures: (A) 303 K; (B) 318 K; (C) 333 K; and (D) 348 K.

The capacitance (C) of a nanoparticle is related to U by

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and is size dependent. Typical values of capacitances of nanocrystals are in the range of 10-18F (aF). At such low capacitances, successive charging events are no longer continuous but are discrete. A measurable change in potential is brought about by varying the charge on the nanocrystal by e. This is often seen as a coulomb staircase in the I-V spectra of individual nanocrystals. We have been able to identify the size-dependent changes in the coulomb staircase phenomena in Au and Pd nanocrystals (see Fig. 6).[29] It has been proposed that, by using nanocrystals, single-electron devices such as supersensitive electrometers and memory devices could be fabricated. Further, it is supposed that nano-objects could bring a new era in electronics, aptly named nanoelectronics. Proof of concept experiments to test the feasibility of using chemically prepared nanocrystals in single-electron devices have been carried out. For example, Murray and coworkers have found that a single redox reaction taking place at the surface of Au nanocrystals induces a eightfold increase in capacitance.[30] Single-electron transistors have been demonstrated with one or a few nanocrystals at the gap between electrodes.[31] Schffrin and coworkers have fabricated a nanoswitch based on a layer of Au nanoparticles on a viologen moiety anchored to Au substrate. The I-V characteristics of the Au nanoparticles studied using insitu STS revealed a dependence on the redox state of the viologen groups underneath the nanoparticle. By electrochemically altering the redox state, the conductivity of the circuit could be made high or low.[32]

I-V characteristics of dodecanethiol thiol capped Au nanocrystals deposited on an ordered monolayer of gadolinium stearate molecules on a Si substrate.

Fig. 5 I-V characteristics of dodecanethiol thiol capped Au nanocrystals deposited on an ordered monolayer of gadolinium stearate molecules on a Si substrate.

 (A) I-V characteristics of an isolated 3.3 nm Pd nanocrystal (dotted line) and the theoretical fit (solid line) obtained at 300 K using a semiclassical model according to which the observed capacitance (C) can be resolved into two components C1 and C2, and the resistance (R) into R1 and R2, such that C = C1 + C2 and R = R1 + R2. For C1 < C2 and R1 < R2, the model predicts steps in the measured current to occur at critical voltages, Vc = ne + qo "Ce=2, where q0 is the residual charge. (B) Variation of the charging energies of Pd and Au nanocrystals with inverse diameters (d).

Fig. 6 (A) I-V characteristics of an isolated 3.3 nm Pd nanocrystal (dotted line) and the theoretical fit (solid line) obtained at 300 K using a semiclassical model according to which the observed capacitance (C) can be resolved into two components C1 and C2, and the resistance (R) into R1 and R2, such that C = C1 + C2 and R = R1 + R2. For C1 < C2 and R1 < R2, the model predicts steps in the measured current to occur at critical voltages, Vc = ne + qo "Ce=2, where q0 is the residual charge. (B) Variation of the charging energies of Pd and Au nanocrystals with inverse diameters (d).

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