Light Quasiparticles Dominate Electronic Transport in Molecular Crystal Field-Effect Transistors

Li, Z. Q.; Podzorov, Vitaly; Sai, Na; Martin, Michael; Gershenson, M. E.; Ventra, Massimiliano Di; Basov, D. N.

doi:10.1103/physrevlett.99.016403

Cited by 133 publications

(129 citation statements)

References 20 publications

(33 reference statements)

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“…Despite the experimental difficulties, there have been several reports of the optical conductivity of excess carriers in rubrene. Measurements by different groups [37,38,24] agree in showing a markedly non-Drude behavior at room temperature: the optical conductivity exhibits a finite frequency maximum in the far infra-red range, indicating a breakdown of semiclassical behavior, which has been ascribed to some form of localization of the charge carriers. [38] We will come back to this crucial observation in Sec.…”

Section: Experimental Overviewmentioning

confidence: 81%

The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

Fratini

Mayou

Ciuchi

2016

Adv Funct Materials

338

452

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Charge transport in crystalline organic semiconductors is intrinsically limited by the presence of large thermal molecular motions, which are a direct consequence of the weak van der Waals inter-molecular interactions. These lead to an original regime of transport called transient localization, sharing features of both localized and itinerant electron systems. After a brief review of experimental observations that pose a challenge to the theory, we concentrate on a commonly studied model which describes the interaction of the charge carriers with inter-molecular vibrations. We present different theoretical approaches that have been applied to the problem in the past, and then turn to more modern approaches that are able to capture the key microscopic phenomenon at the origin of the puzzling experimental observations, i.e. the quantum localization of the electronic wavefuntion at timescales shorter than the typical molecular motions. We describe in particular a relaxation time approximation which clarifies how the transient localization due to dynamical molecular motions relates to the Anderson localization realized for static disorder, and allows us to devise strategies to improve the mobility of actual compounds. The relevance of the transient localization scenario to other classes of systems is briefly discussed.

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Section: Experimental Overviewmentioning

confidence: 81%

The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

Fratini

Mayou

Ciuchi

2016

Adv Funct Materials

338

452

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“…In our analysis of these multilayer structures, we followed the protocol detailed in ref. 27. Specifically, we carried out reflection, transmission and ellipsometric measurements on the Si substrates and SiO 2 /Si wafers used in our devices and thus obtained the optical constants of layers 2 and 4.…”

Section: Extracting the Optical Constants Of Graphenementioning

confidence: 99%

“…1. Finally, we used a multi-oscillator fitting procedure 27 to account for the contribution of σ(ω) of graphene to the reflectance and transmission spectra shown in Fig. 1 using standard methods for multilayered structures.…”

Section: Extracting the Optical Constants Of Graphenementioning

confidence: 99%

Dirac charge dynamics in graphene by infrared spectroscopy

et al. 2008

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A remarkable manifestation of the quantum character of electrons in matter is offered by graphene, a single atomic layer of graphite. Unlike conventional solids where electrons are described with the Schrödinger equation, electronic excitations in graphene are governed by the Dirac hamiltonian 1 . Some of the intriguing electronic properties of graphene, such as massless Dirac quasiparticles with linear energy-momentum dispersion, have been confirmed by recent observations 2-5 . Here, we report an infrared spectromicroscopy study of charge dynamics in graphene integrated in gated devices. Our measurements verify the expected characteristics of graphene and, owing to the previously unattainable accuracy of infrared experiments, also uncover significant departures of the quasiparticle dynamics from predictions made for Dirac fermions in idealized, freestanding graphene. Several observations reported here indicate the relevance of many-body interactions to the electromagnetic response of graphene.We investigated the reflectance R(ω) and transmission T (ω) of graphene samples on a SiO 2 /Si substrate (inset of Fig. 1a) as a function of gate voltage V g at 45 K (see the Methods section). We start with data taken at the charge-neutrality point V CN : the gate voltage corresponding to the minimum d.c. conductivity and zero total charge density (inset of Fig. 1c). Figure 1a shows R(ω) of a graphene gated structure (graphene/SiO 2 /Si) at V CN = 3 V normalized by reflectance of the substrate R sub (ω). R sub (ω) is dominated by a minimum around 5,500 cm −1 due to interference effects in SiO 2 . A remarkable observation is that a monolayer of undoped graphene markedly modifies the interference minimum of the substrate leading to a suppression of R sub (ω) by as much as 15%. This observation is significant because it enables us to evaluate the conductivity of graphene near the interference structure, as will be discussed below.Both reflectance and transmission spectra of graphene structures can be modified by a gate voltage. Figure 1b,c shows these modifications at various gate voltages normalized by data atThese data correspond to the Fermi energy E F on the electron side and similar behaviour was observed with E F on the hole side (not shown). At low voltages (<17 V), we found a dip in R(V )/R(V CN ) spectra. With increasing bias, this feature evolves into a peak-dip structure and systematically shifts to higher frequency. The T (V )/T (V CN ) spectra reveal a peak at all voltages, which systematically hardens with increasing bias. A voltage-induced increase in transmission (T (V )/T (V CN ) > 1) signals a decrease of the absorption with bias. Most interestingly, we observed that the frequencies of the main features in R(V )/R(V CN ) and T (V )/T (V CN ) all evolve approximately as √ V . To explore the quasiparticle dynamics under applied voltages, it is imperative to first discuss the two-dimensional (2D) optical conductivity of charge-neutral graphene, σ 1 (ω, V CN ) + iσ 2 (ω, V CN ), extracted from a multilayer analy...

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“…[5][6][7][8][9] In analogy to inorganic semiconductors, this has been interpreted as an indication of delocalized charge carriers and a "bandlike" transport mechanism. [10][11][12][13] Theoretical models based on polaronic band conduction 14 indeed provide a good qualitative description of the bandlike mobility of charge carriers in pentacene, but the factors determining the absolute charge mobility remain unknown. For instance, an analysis of the high temperature mobility has indicated that the mean free path of charge carriers is of the same magnitude as the intermolecular distance.…”

mentioning

confidence: 99%

On the mechanism of charge transport in pentacene

Laarhoven

Flipse

Koeberg³

et al. 2008

The Journal of Chemical Physics

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Terahertz transient conductivity measurements are performed on pentacene single crystals, which directly demonstrate a strong coupling of charge carriers to low frequency molecular motions with energies centered around 1.1 THz. We present evidence that the strong coupling to low frequency motions is the factor limiting the conductivity in these organic semiconductors. Our observations explain the apparent paradox of the "bandlike" temperature dependence of the conductivity beyond the validity limit of the band model. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2955462͔Recent years have seen much interest in the conductivity of pentacene and other molecular semiconductors of the oligoacene family, fueled by the unique properties of these materials. Oligoacenes are flexible, cheap, and exhibit relatively high charge carrier mobilities, 1 allowing for novel applications in electronic devices. [2][3][4] Despite this interest, the nature of charge transport in the materials has remained the subject of intense debate.Crystalline organic semiconductors exhibit charge mobilities that decrease with temperature. [5][6][7][8][9] In analogy to inorganic semiconductors, this has been interpreted as an indication of delocalized charge carriers and a "bandlike" transport mechanism. 10-13 Theoretical models based on polaronic band conduction 14 indeed provide a good qualitative description of the bandlike mobility of charge carriers in pentacene, but the factors determining the absolute charge mobility remain unknown. For instance, an analysis of the high temperature mobility has indicated that the mean free path of charge carriers is of the same magnitude as the intermolecular distance. 15 It is not clear as to why the signature of delocalized bandlike transport persists at these temperatures, where the mean free path is smaller than the intermolecular distances. Because of the "softness" of these van der Waals bonded organic crystals, it is possible that the coupling of the carriers with the intermolecular vibrations could play a key role in the description of the charge carrier dynamics. Troisi and Orlandi 16 have recently formulated a model ͑TO model͒ that states that the charge mobility in oligoacenes is limited by large fluctuations of the electronic coupling between adjacent organic molecules due to thermal molecular motions, in particular, the low energetic phonon modes with energies around 1 THz. 17 This model correctly describes the bandlike temperature dependence of the mobility, including its absolute value, outside the validity of delocalized transport models. 16 Direct experimental evidence for the various proposed models has been lacking. In particular, experimental insights into the role of intermolecular low frequency vibrational modes in determining the charge transport mechanism have been missing to date.Terahertz spectroscopy 18 provides the opportunity of studying the response of charge carriers in the frequency range of these modes, and has proven useful for investigating photocarrier dynamics in...

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Light Quasiparticles Dominate Electronic Transport in Molecular Crystal Field-Effect Transistors

Cited by 133 publications

References 20 publications

The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

Dirac charge dynamics in graphene by infrared spectroscopy

On the mechanism of charge transport in pentacene

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