Prospects of Colloidal Nanocrystals for Electronic - Computer Science

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410 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al. are maximally disordered. For fields beyond Hc, the resistance again decreases to the initial value 335 (Figure 25b). With increasing temperature above 2 K, MR decreases rapidly due to spin-flip scattering, down to ∼10% of its maximum value at 20 K. At very low temperatures, the cotunneling processes can result in significant enhancement of MR. 339 Larger values of MR can be achieved with the class of materials called half-metals. A half-metal is characterized by the presence of an energy gap at the Fermi level for only one electron spin direction, while the energy band for the opposite spin direction is continuous. 340 Such band structure leads to P ) 1 and theoretical ∆R/Rmax values of 100% and 50% for antiparallel and random spin orientations at H ) 0, correspondingly. Magnetite Fe3O4 is the example of a halfmetal. Recently, Zeng et al. reported appreciable 20-35% MR values at T ) 60 K for a superlattice of 6 nm diameter Fe3O4 NCs. 341 (Figure 25c). At room temperature, MR of Fe3O4 NC solids remained as large as 12%. For comparison, room temperature MR of magnetite thin films does not exceed a few percent. 341 The presence of a very large number of tunneling junctions in a weakly coupled NC solid has several advantageous effects on spin-dependent electronic transport. Because current through the NC array is carried by multiple parallel conduction paths containing different numbers of NCs, the device MR should be less sensitive to applied bias voltage. 342 Indeed, both Co and Fe3O4 NC solids demonstrated weak dependence of magnetoresistance on the applied voltage, in contrast to thin film devices that often show steep reduction in MR with increasing bias. 334,341 This advantage may help NC arrays to find use in magnetic recording heads and nonvolatile magnetoresistive random access memory (MRAM) applications. 343 However, further developments in understanding and improving spin-dependent electronic properties are necessary to successfully compete with more mature technologies based on vacuum-deposited thin films and heterostructures. The possibility of combining magnetic NCs with other materials, for example, semiconductor quantum dots in form of multicomponent nanostructures (Figure 6) or binary superlattices (Figure 11), may lead to interesting multifunctional electronic materials. 5.6. Semiconductor Nanocrystals: Electronic Structure and Shell Filling Semiconductor NCs represent probably the most interesting and important class of inorganic solution processed electronic materials. They have a real chance to find use in the next generations of large area solar cells and lightemitting devices. Fundamental understanding of charge transport is also crucial for developments of NC-based electrically pumped lasers, low-dimensional thermoelectric materials, 254 photodetectors, 265,294 and many other technologies. The electronic structure of semiconductor NCs is dictated by strong quantum confinement, which gives rise to discrete electron and hole states called quantum confined orbitals (Figure 26a). Their energies are directly related to the NC size and shape. 3,39 In a spherical NC, the quantum confinement leads to the series of electron and hole states with S, P, D, and F symmetries identical to the energy levels of a hydrogen atom (Figure 26b). The electron and hole states will be further labeled with “e” and “h” indexes, respectively. The symmetry of NC atomic lattice determines the degeneracy of the quantum confined states. Thus, NCs with wurtzite and zinc blend lattices typical for most II-VI and III-V semiconductors have 2-, 6-, and 10-fold degenerated 1Se, 1Pe, and 1De states, respectively, 3 whereas PbS, PbSe, and PbTe NCs with rock salt lattices have 4 times higher degeneracy and can accommodate up to eight electrons on their 1Se and 1Sh states. 344 In a neutral NC, the highest occupied (1Sh) and lowest unoccupied (1Se) states are separated by the forbidden energy gap that is much larger than kBT (∼25 meV at 300 K), and electrons cannot be thermally excited into 1Se state. Semiconductor NCs do not contain conduction electrons and holes in the neutral ground state, and additional carriers should be added or generated by, for example, photoexcitation to make the NCs solid conductive. The conductivity of a NC solid depends on the number of conduction electrons or holes per NC and their mobility, which in turn is determined by the tunneling rate discussed above. In strongly confined NCs, the gaps between S, P, and D states exceed thermal energy, and additional carriers sequentially occupy the quantum confined states following the Pauli principle319,320 as shown in Figure 26c. Banin et al. directly observed sequential filling of the quantum confined states in NC quantum dots using scanning tunneling spectroscopy. 345 Addition of each electron to the NC costs Coulombic charging energy (Ec) and the energy of electrostatic repulsion between the incoming electron and the additional electrons already present in the NC (Ee-e). These terms lift degeneracy of the S, P, and D states320 Figure 26. (a) Size-dependent electronic structure of individual semiconductor nanocrystal. Because of strong quantum confinement, the continuum of electronic states in the valence and conduction bands collapses into a set of discrete states corresponding to the atomic-like S, P, and D orbitals shown in (b). (c) The electrochemical potentials for sequential additions of electrons (indicated in gray) to a typical semiconducotor nanocrystal. The first and second electron is added to the S orbitals, the third to eighth electron to the P orbitals. The Coulombic charging energy lifts degeneracy of S and P states. Adopted with permission from ref 319. Copyright Royal Chemical Society. (Figure 26c). 5.7. Doping of Semiconductor Nanoparticles and Nanoparticle Assemblies Numerous early studies have revealed low electronic conductivity of semiconductor NC solids because of negli-
Colloidal Nanocrystals in Electronic Applications Chemical Reviews, 2010, Vol. 110, No. 1 411 gibly small concentration of mobile carriers and the presence of traps associated with dangling bonds at NC surfaces. 263,346,347 The attempts to increase the concentration of mobile carriers in NC solids by substitutional doping, that is, by adding atomic impurities with larger or smaller number of valence electrons as compared to the atoms in the host lattice, demonstrated only limited success 348 for several reasons. First, the incorporation of atomic impurities in NCs was found to be difficult because of the lack of appropriate chemical techniques and the effect of self-purification; the impurities could easily anneal away from the NC due to its tiny size. 348,349 Moreover, the incorporation of a heterovalent impurity does not guarantee stable electronic doping of a NC. In a bulk material, the electronic dopant forms a shallow trap state that lies within a few meVs of the corresponding band edge. This alignment can be easily distorted by the quantum confinement; the depth of impurity level (δE) that determines the probability to thermally excite carrier into the conduction 1S state changes with particle size. 348 The trap depth can increase when the NC band gap becomes wider in response to the quantum confinement. 350 At the same time, when the NC size becomes comparable with ϑ ) [ε/(m*/m0)]aB, where aB is the hydrogen Bohr radius and m0 is the free electron mass, the shallow trap itself starts feeling the confinement potential and shifts to higher energy. 3 For sizes smaller than ϑ, the quantum confinement blurs the distinction between shallow traps and the Fourier transform states (quantum confined orbitals); the impurities autoionize and provide carriers to the crystallite. 348 These competitive trends result in complicating the size dependence of δE. Because substitutional doping of NCs and NC solids is somewhat complicated, other approaches have been explored. Yu et al. reported that the conductivity of CdSe NC solids increased by about 3 orders of magnitude after exposure to potassium vapor in high vacuum. 185 It was shown that deposited potassium atoms donate electrons to 1Se states of CdSe NCs. The carriers can also be injected in the NC solid by electrochemical charging. Charge screening by mobile ions present in the electrolyte substantially reduced Ec, facilitating charging of a NC solid with several additional electrons per NC 3,351 (Figure 27). Electrochemical charging allowed precise tuning of the electrochemical potential and controlling the doping level of NC solid. Guyot-Sionnest et al. demonstrated sequential filling of 1Se and 1Pe orbitals by electrochemical charging and studied carrier transport through S and P states of quantum confined CdSe NC solids 185,352,353 (Figure 27a,b). The filling of the quantum confined states was accompanied by the characteristic maxima and minima of electron differential mobility corresponding to half- and completely filled S- and P-states of the NCs (Figure 27b). The electron transfer rate from an occupied to empty state is proportional to � (1 - �)Γ, where � is the occupation fraction. It maximizes at � ) 0.5, leading to the conduction maximum for the half-filled shell. 185 The complete filling of S or P shells (� ) 1) creates transport bottleneck. In this situation, conduction is only possible by exciting the electron to the next unoccupied shell. 185 To confirm that injected charge carriers indeed occupy the quantum confined states rather than being trapped at the NC surface, Guyot-Sionnest et al. monitored the evolution of optical absorption spectra during electrochemical charging of the NC solid 185,352-354 (Figure 27c). The presence of electrons at the 1Se or 1Pe quantum confined states should lead to reduction of the absorption cross section for interband (e.g., 1Sh(3/2)-1Se) excitonic transitions, so-called “optical bleaching” of the excitonic peaks (Figure 27c). In addition, new absorption bands corresponding to the intraband (e.g., 1Se-1Pe) optical transitions should appear in the mid-IR region. Nanocrystals and NC solids can also be doped via charge transfer from other chemical species. The treatment of CdSe, CdS, or ZnO NCs with sodium biphenyl results in the Figure 27. Electrochemical doping and charge transport in CdSe nanocrystal solids. (a) Cyclic voltammetry of a TOP/TOPO-capped 6.4 nm CdSe nanocrystal solid treated with 1,7-heptanediamine. The arrows indicate the cycle direction. The dotted line (linear scale) is the electrochemical current, and the solid line (logarithmic scale) is the current proportional to the conductivity of the nanocrystal film. (b) Additional charge injected into a film of 6.4 nm CdSe nanocrystals by electrochemical charging (dotted line, linear scale) leads to the variations in and electron differential mobility (solid line, logarithmic scale) with maxima and minima corresponding to half- and fully occupied S and P quantum confined orbitals. (c) The correlation of conductance and optical bleach of the excitonic peaks for a film of 5.4-nm CdSe nanocrystals. The solid line represents the conductance. The “O” represent the bleach magnitude at the 1S exciton peak (593 nm), and the “0” represent the bleach magnitude at the 1P exciton (520 nm). The first exciton is completely bleached at the most negative potentials. The potentials are measured against the Ag pseudoreference electrode. Reprinted with permission from ref 185. Copyright 2003 American Association for Advancement of Science.
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