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Lateral homogeneity of the electronic properties in pristine and ion-irradiated graphene probed by scanning capacitance spectroscopy.

Giannazzo F, Sonde S, Rimini E, Raineri V - Nanoscale Res Lett (2011)

Bottom Line: In this article, a scanning probe method based on nanoscale capacitance measurements was used to investigate the lateral homogeneity of the electron mean free path both in pristine and ion-irradiated graphene.The local variations in the electronic transport properties were explained taking into account the scattering of electrons by charged impurities and point defects (vacancies).The local density of the charged impurities and vacancies were determined for different irradiated ion fluences.

View Article: PubMed Central - HTML - PubMed

Affiliation: CNR-IMM, Strada VIII, 5, Zona Industriale, 95121, Catania, Italy. filippo.giannazzo@imm.cnr.it.

ABSTRACT
In this article, a scanning probe method based on nanoscale capacitance measurements was used to investigate the lateral homogeneity of the electron mean free path both in pristine and ion-irradiated graphene. The local variations in the electronic transport properties were explained taking into account the scattering of electrons by charged impurities and point defects (vacancies). Electron mean free path is mainly limited by charged impurities in unirradiated graphene, whereas an important role is played by lattice vacancies after irradiation. The local density of the charged impurities and vacancies were determined for different irradiated ion fluences.

No MeSH data available.


Evaluation of the effective area from local capacitance measurements. Local capacitance-voltage curves measured on fixed positions on bare SiO2 (a) and on graphene-coated SiO2 (b) for a sample not subjected to ion irradiation. AFM morphology of a graphene flake on SiO2, with indicated the probed positions by the SCS tip. (inset of a). Effective area evaluated from the C-V curves in (a) and (b). Schematic representation of Atip and Aeff (inset of c).
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Figure 2: Evaluation of the effective area from local capacitance measurements. Local capacitance-voltage curves measured on fixed positions on bare SiO2 (a) and on graphene-coated SiO2 (b) for a sample not subjected to ion irradiation. AFM morphology of a graphene flake on SiO2, with indicated the probed positions by the SCS tip. (inset of a). Effective area evaluated from the C-V curves in (a) and (b). Schematic representation of Atip and Aeff (inset of c).

Mentions: In Figure 2, capacitance-voltage curves measured on fixed positions on bare SiO2 and on graphene-coated SiO2 are reported for a sample not subjected to ion irradiation. The tip positions are indicated in the AFM image in the inset of Figure 2a. When the tip is in contact on bare SiO2, a typical capacitance-voltage curve for a metal-oxide-semiconductor (MOS) capacitor from accumulation (at negative sample bias) to depletion (at positive sample bias) is measured (see Figure 2a). The area of the MOS capacitor is represented by the tip contact area Atip, as illustrated in the insert of Figure 2c. When tip is in contact on graphene, the measured capacitance is minimum around zero bias and increases both for negative and positive bias (see Figure 2b). At Vg = 0, the Fermi level in graphene is almost coincident with the Dirac point. A positive modulating bias between the substrate and the tip locally induces a shift of the graphene quasi-Fermi energy EF in the conduction band, and, hence, an accumulation of electrons at the nanometric tip/graphene contact. On the contrary, a negative bias induces a shift of EF in the valence band, and, hence, an accumulation of holes at the tip/graphene contact. The carrier density n induced by the gate bias Vg can be expressed as n = Cox'Vg/q, where q is the electron charge, and Cox' is the oxide capacitance per unit area (Cox' = εoxε0/tox, being ε0 the vacuum permittivity, εox = 3.9 and tox are the relative permittivity and the thickness of the SiO2 film, respectively). The value of EF can be related to the applied bias as EF = ħvFkF, being kF = (πn)1/2, ħ the reduced Planck's constant, and vF = 1 × 106 m/s, the electron Fermi velocity in graphene. The induced charge n spreads over an area, Aeff, which can be thought as the tip-graphene-insulator-semiconductor capacitor effective area (as schematically illustrated in the insert of Figure 2c). The effective area Aeff can be evaluated from the ratio of the capacitance measured with the probe on graphene-coated regions (/ΔCgr/) and on bare SiO2 regions (/ΔCox/) [15], i.e., Aeff = Atip/ΔCgr///ΔCox/, where the tip contact area Atip can be independently determined by scanning electron microscopy (Atip = 80 nm2 in the present case). The evaluated Aeff is reported as a function of the gate bias in Figure 2c. Except for Vg = 0, Aeff increases linearly with /Vg/ both for negative and positive Vg values.


Lateral homogeneity of the electronic properties in pristine and ion-irradiated graphene probed by scanning capacitance spectroscopy.

Giannazzo F, Sonde S, Rimini E, Raineri V - Nanoscale Res Lett (2011)

Evaluation of the effective area from local capacitance measurements. Local capacitance-voltage curves measured on fixed positions on bare SiO2 (a) and on graphene-coated SiO2 (b) for a sample not subjected to ion irradiation. AFM morphology of a graphene flake on SiO2, with indicated the probed positions by the SCS tip. (inset of a). Effective area evaluated from the C-V curves in (a) and (b). Schematic representation of Atip and Aeff (inset of c).
© Copyright Policy - open-access
Related In: Results  -  Collection

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Figure 2: Evaluation of the effective area from local capacitance measurements. Local capacitance-voltage curves measured on fixed positions on bare SiO2 (a) and on graphene-coated SiO2 (b) for a sample not subjected to ion irradiation. AFM morphology of a graphene flake on SiO2, with indicated the probed positions by the SCS tip. (inset of a). Effective area evaluated from the C-V curves in (a) and (b). Schematic representation of Atip and Aeff (inset of c).
Mentions: In Figure 2, capacitance-voltage curves measured on fixed positions on bare SiO2 and on graphene-coated SiO2 are reported for a sample not subjected to ion irradiation. The tip positions are indicated in the AFM image in the inset of Figure 2a. When the tip is in contact on bare SiO2, a typical capacitance-voltage curve for a metal-oxide-semiconductor (MOS) capacitor from accumulation (at negative sample bias) to depletion (at positive sample bias) is measured (see Figure 2a). The area of the MOS capacitor is represented by the tip contact area Atip, as illustrated in the insert of Figure 2c. When tip is in contact on graphene, the measured capacitance is minimum around zero bias and increases both for negative and positive bias (see Figure 2b). At Vg = 0, the Fermi level in graphene is almost coincident with the Dirac point. A positive modulating bias between the substrate and the tip locally induces a shift of the graphene quasi-Fermi energy EF in the conduction band, and, hence, an accumulation of electrons at the nanometric tip/graphene contact. On the contrary, a negative bias induces a shift of EF in the valence band, and, hence, an accumulation of holes at the tip/graphene contact. The carrier density n induced by the gate bias Vg can be expressed as n = Cox'Vg/q, where q is the electron charge, and Cox' is the oxide capacitance per unit area (Cox' = εoxε0/tox, being ε0 the vacuum permittivity, εox = 3.9 and tox are the relative permittivity and the thickness of the SiO2 film, respectively). The value of EF can be related to the applied bias as EF = ħvFkF, being kF = (πn)1/2, ħ the reduced Planck's constant, and vF = 1 × 106 m/s, the electron Fermi velocity in graphene. The induced charge n spreads over an area, Aeff, which can be thought as the tip-graphene-insulator-semiconductor capacitor effective area (as schematically illustrated in the insert of Figure 2c). The effective area Aeff can be evaluated from the ratio of the capacitance measured with the probe on graphene-coated regions (/ΔCgr/) and on bare SiO2 regions (/ΔCox/) [15], i.e., Aeff = Atip/ΔCgr///ΔCox/, where the tip contact area Atip can be independently determined by scanning electron microscopy (Atip = 80 nm2 in the present case). The evaluated Aeff is reported as a function of the gate bias in Figure 2c. Except for Vg = 0, Aeff increases linearly with /Vg/ both for negative and positive Vg values.

Bottom Line: In this article, a scanning probe method based on nanoscale capacitance measurements was used to investigate the lateral homogeneity of the electron mean free path both in pristine and ion-irradiated graphene.The local variations in the electronic transport properties were explained taking into account the scattering of electrons by charged impurities and point defects (vacancies).The local density of the charged impurities and vacancies were determined for different irradiated ion fluences.

View Article: PubMed Central - HTML - PubMed

Affiliation: CNR-IMM, Strada VIII, 5, Zona Industriale, 95121, Catania, Italy. filippo.giannazzo@imm.cnr.it.

ABSTRACT
In this article, a scanning probe method based on nanoscale capacitance measurements was used to investigate the lateral homogeneity of the electron mean free path both in pristine and ion-irradiated graphene. The local variations in the electronic transport properties were explained taking into account the scattering of electrons by charged impurities and point defects (vacancies). Electron mean free path is mainly limited by charged impurities in unirradiated graphene, whereas an important role is played by lattice vacancies after irradiation. The local density of the charged impurities and vacancies were determined for different irradiated ion fluences.

No MeSH data available.