Metal-insulator transition in a semiconductor nanocrystal network

Abstract

Many envisioned applications of semiconductor nanocrystals (NCs), such as thermoelectric generators and transparent conductors, require metallic (nonactivated) charge transport across an NC network. Although encouraging signs of metallic or near-metallic transport have been reported, a thorough demonstration of nonzero conductivity, σ, in the 0 K limit has been elusive. Here, we examine the temperature dependence of σ of ZnO NC networks. Attaining both higher σ and lower temperature than in previous studies of ZnO NCs (T as low as 50 mK), we observe a clear transition from the variable-range hopping regime to the metallic regime. The critical point of the transition is distinctly marked by an unusual power law close to σ ∝ T1/5. We analyze the critical conductivity data within a quantum critical scaling framework and estimate the metal-insulator transition (MIT) criterion in terms of the free electron density, n, and interparticle contact radius, ρ.

Contact Conductance Governs Metallicity in Conducting Metal Oxide Nanocrystal Films

“…Metallic nanoparticles, including elemental metals and doped semiconductors, hold promise as solution-processable building blocks to prepare conducting materials, from metallic interconnects to transparent conducting films and electrocatalytic aerogels. − But such materials exhibit wide-ranging transport properties and often their conductivity is thermally activated, implying they are in fact insulators. − Unless the nanocrystals are fused into bulk metals, for example, by thermal annealing, conventional metallic behavior has rarely been observed. , In such cases, conductivity decreases as temperature increases since phonon scattering limits the electron mobility. Between these two limits, nanostructured materials may be metallic, meaning their conductivity approaches a finite, nonzero value in the low-temperature limit, but still have an unusual negative thermal coefficient of resistivity (TCR). ,− Although strengthening electronic coupling, by using solution- or vapor-phase deposition to fill the spaces between metal oxide nanocrystals ,, or replacing long insulating ligands with shorter ones, ,, is known to increase conductivity, criteria determining the temperature-dependent transport behavior remain the subject of intense investigation. − …”

“…8−13 Unless the nanocrystals are fused into bulk metals, for example, by thermal annealing, conventional metallic behavior has rarely been observed. 2,9 In such cases, conductivity decreases as temperature increases since phonon scattering limits the electron mobility. Between these two limits, nanostructured materials may be metallic, meaning their conductivity approaches a finite, nonzero value in the low-temperature limit, but still have an unusual negative thermal coefficient of resistivity (TCR).…”

“…Between these two limits, nanostructured materials may be metallic, meaning their conductivity approaches a finite, nonzero value in the low-temperature limit, but still have an unusual negative thermal coefficient of resistivity (TCR). 9,14−16 Although strengthening electronic coupling, by using solution-or vapor-phase deposition to fill the spaces between metal oxide nanocrystals 2,9,11 or replacing long insulating ligands with shorter ones, 6,17,18 is known to increase conductivity, criteria determining the temperaturedependent transport behavior remain the subject of intense investigation. 8−11 Establishing clear criteria to predict electronic properties has been challenging in part due to limitations in tuning physical parameters that determine conductivity, such as nanocrystal size, electron concentration, and the strength of electronic coupling between nanocrystals.…”

Nanocrystal‐based inorganic nanocomposites: A new paradigm for plasma‐produced optoelectronic thin films

“…Greenberg et al [ 46 ] expanded on these studies. They showed that the electronic properties of plasma‐deposited ZnO nanocrystal films (Figure 3a,b) could be modified to enable metallic conduction by applying the following measures, in part illustrated in Figure 3c: (a) photonic sintering through intense pulsed light exposure to grow the nanocrystal‐nanocrystal contact facet; (b) ALD deposition of ZnO to further grow the contact facet radius; (c) ALD of Al 2 O 3 to remove electron trap states on the ZnO network; and (d) additional photodoping to raise the carrier concentration.…”

“…Curves above the green curve represent metallic conduction, below the green curve hopping conduction. (a–d) Reproduced with permission from Greenberg et al [ 46 ]…”