Band-like transport, high electron mobility and high photoconductivity in all-inorganic nanocrystal arrays

Lee, Jong Soo; Kovalenko, Maksym V.; Huang, Jing; Chung, Dae Sung; Talapin, Dmitri V.

doi:10.1038/nnano.2011.46

Cited by 692 publications

(578 citation statements)

References 25 publications

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“…Substituting the conductivity from Eq. (3) into the expression µ = 6G/πend (where the factor 6/π accounts for the difference between the concentration of electrons inside NCs and the average concentration in the film) we find the low-temperature metallic mobility µ = 3 5/3 2π 2/3 e ρ 2 g 2/3 n 1/3 d .For CdSe NCs with d = 4 nm, µ is on the order of 10 cm 2 /V · s at n = 2n c and is close to the experimentally observed room-temperature mobility of 30 cm 2 /V · s for CdSe [26][27][28]. Note that this mobility mostly is due to the contact resistance, while the in the case of bulk semiconductors the low temperature mobility is due to the scattering by donors.…”

supporting

confidence: 84%

Metal–insulator transition in films of doped semiconductor nanocrystals

Chen¹,

Reich²,

Kramer³

et al. 2015

Nature Mater

109

134

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To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration nc for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted nc, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition.Semiconductor nanocrystals (NCs) have shown great potential in optoelectronics applications such as solar cells [1], light emitting diodes [2], and field-effect transistors [3,4] by virtue of their size-tunable optical and electrical properties [5] and low-cost solution-based processing techniques [6,7]. These applications require conducting NC films and the introduction of extra carriers through doping can enhance the electrical conduction. Several strategies for NC doping have been developed. Remote doping, the use of suitable ligands as donors in the vicinity of NC surface, increased the conductivity of PbSe NC films by 12 orders of magnitude [8]. Electrochemical doping, which tunes the carrier concentration accurately and reversibly, resulted in conducting NC films [9,10]. Lately, stoichiometric control has emerged as a strategy to dope lead chalcogenide NCs [11]. Finally, electronic impurity doping of NCs, originally impeded by synthetic challenges [12], was recently achieved in InAs [13] and CdSe [14] NCs.While many experimental studies have been directed towards increasing the conductivity of NC films, there is still no clear consensus on the fundamental question: what is the condition for the metal-insulator transition (MIT) in NC films [15][16][17]? In a bulk semiconductor, the critical electron concentration n M for the MIT depends on the Bohr radius a B according to the well-known Mott criterion [18] n M a 3 B 0.02, where a B = ε 2 /m * e 2 is the effective Bohr radius (in Gaussian units), ε is the dielectric constant of the semiconductor, and m * is the effective electron mass. It is obvious that a dense film of undoped semiconductor NCs is an insulator, while a film of touching metallic NCs with the same geometry is a conductor. Therefore, the MIT has to occur in semiconductor NC films at some criti-FIG. 1. The origin of the metal-to-insulator transition in semiconductor nanocrystal films. The figure shows the cross section of two nanocrystals in contact through facets with radius ρ. The blue spherical cloud represents an electron wave packet which moves through the contact. Such a compact wave pac...

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supporting

confidence: 84%

Metal–insulator transition in films of doped semiconductor nanocrystals

Chen¹,

Reich²,

Kramer³

et al. 2015

Nature Mater

109

134

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“…NCAs with long organic ligands are electrically insulating, inhibiting electrons as potential heat carriers. NCAs with short inorganic ligands, however, have such a small inter-core separation that electronic coupling between neighbouring nanocrystals has been reported 6 . Using the WiedemannFranz law for PbSe-N 2 H 4 and CdSe-In 2 Se 4 2− NCAs (refs 5,6), we estimate electronic thermal conductivities of 6.2 × 10 −3 W m −1 K −1 and 2.7 × 10 −2 W m −1 K −1 based on their electrical conductivities.…”

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confidence: 99%

Surface chemistry mediates thermal transport in three-dimensional nanocrystal arrays

et al. 2013

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Arrays of ligand-stabilized colloidal nanocrystals with sizetunable electronic structure are promising alternatives to single-crystal semiconductors in electronic, optoelectronic and energy-related applications 1-5 . Hard/soft interfaces in these nanocrystal arrays (NCAs) create a complex and uncharted vibrational landscape for thermal energy transport that will influence their technological feasibility. Here, we present thermal conductivity measurements of NCAs (CdSe, PbS, PbSe, PbTe, Fe 3 O 4 and Au) and reveal that energy transport is mediated by the density and chemistry of the organic/inorganic interfaces, and the volume fractions of nanocrystal cores and surface ligands. NCA thermal conductivities are controllable within the range 0.1-0.3 W m −1 K −1 , and only weakly depend on the thermal conductivity of the inorganic core material. This range is 1,000 times lower than the thermal conductivity of silicon, presenting challenges for heat dissipation in NCA-based electronics and photonics. It is, however, 10 times smaller than that of Bi 2 Te 3 , which is advantageous for NCA-based thermoelectric materials.Colloidal nanocrystals self-assemble into NCAs with electronic and optical properties that can be broadly tuned by nanocrystal composition and size 1-4,6-8 . To be considered as viable replacements for traditional semiconductors, NCA-based technologies must also meet thermal management demands as high operating temperatures degrade device performance and lifetime. Thermal conductivity (k) quantifies a material's ability to dissipate heat and relates temperature gradient (∇T ) to heat flux (q) through Fourier's law, q = −k∇T . In metals, most heat is carried by electrons, whereas in semiconductor and insulating crystals, thermal conductivity arises from the transport and scattering of quantized vibrations that are born from the periodic atomic lattice, that is, phonons. Our NCAs are non-metallic and have a vibrational structure that is complicated by compositional heterogeneity and periodicity at two length scales: the atomic lattice within each core and the array of periodic cores separated by ligand monolayers. Studies of planar self-assembled monolayer (SAM) junctions show that surface chemistry can control energy transport at the interface between two solids 9 , but does it also influence the thermal conductivity of a bulk three-dimensional solid with a complex network of internal interfaces? We herein report on the hitherto unknown thermal transport properties of NCAs, using systematic thermal conductivity measurements complemented by heat capacity measurements and atomistic simulations.A series of NCAs was prepared with varying core diameters, core materials and ligand groups (Table 1) through spin-coating concentrated colloidal solutions onto silicon wafers 6 . During film

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“…Although there has been significant progress in developing single high-mobility NC-FETs that further operate with low hysteresis 18,20 at low voltage 19 , these high-performance NC-FETs have not been integrated into NC circuits in the literature. All circuit demonstrations have been limited to two single separate FETs connected externally to form an inverter 19,21 .…”

mentioning

confidence: 99%

“…This greatly expands the applicability of these materials compared with other recently developed novel ligands. For example, although excellent mobilities have been observed with molecular metal chalcogenide complexes 18,19 , these NCs are dissolved in hydrazine, an extremely caustic solvent that is not compatible with flexible plastics. In addition, as a wide range of flexible electronic applications are typically powered by small thin-film batteries or radio frequency fields 22,23 , it is necessary to show the scalability of these colloidal inks to minimize energy consumption.…”

mentioning

confidence: 99%

Flexible and low-voltage integrated circuits constructed from high-performance nanocrystal transistors

et al. 2012

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Colloidal semiconductor nanocrystals are emerging as a new class of solution-processable materials for low-cost, flexible, thin-film electronics. Although these colloidal inks have been shown to form single, thin-film field-effect transistors with impressive characteristics, the use of multiple high-performance nanocrystal field-effect transistors in large-area integrated circuits has not been shown. This is needed to understand and demonstrate the applicability of these discrete nanocrystal field-effect transistors for advanced electronic technologies. Here we report solution-deposited nanocrystal integrated circuits, showing nanocrystal integrated circuit inverters, amplifiers and ring oscillators, constructed from highperformance, low-voltage, low-hysteresis CdSe nanocrystal field-effect transistors with electron mobilities of up to 22 cm 2 V À 1 s À 1 , current modulation 410 6 and subthreshold swing of 0.28 Vdec À 1 . We fabricated the nanocrystal field-effect transistors and nanocrystal integrated circuits from colloidal inks on flexible plastic substrates and scaled the devices to operate at low voltages. We demonstrate that colloidal nanocrystal field-effect transistors can be used as building blocks to construct complex integrated circuits, promising a viable material for low-cost, flexible, large-area electronics.

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Band-like transport, high electron mobility and high photoconductivity in all-inorganic nanocrystal arrays

Cited by 692 publications

References 25 publications

Metal–insulator transition in films of doped semiconductor nanocrystals

Metal–insulator transition in films of doped semiconductor nanocrystals

Surface chemistry mediates thermal transport in three-dimensional nanocrystal arrays

Flexible and low-voltage integrated circuits constructed from high-performance nanocrystal transistors

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