Reversible Photochemical Control of Doping Levels in Supported Graphene

Abstract

Controlling the type and density of charge carriers in graphene is vital for a wide range of applications of this material in electronics and optoelectronics. To date, chemical doping and electrostatic gating have served as the two most established means to manipulate the carrier density in graphene. Although highly effective, these two approaches require sophisticated graphene growth or complex device fabrication processes to achieve both the desired nature and the doping densities with generally limited dynamic tunability and spatial control. Here, we report a convenient and tunable optical approach to tune the steady-state carrier density and Fermi energy in graphene by photochemically controlling the concentration of adsorbed molecular O 2 , a p-dopant in graphene, using femtosecond pulsed laser irradiation in the UV range. As an all-optical approach, it allows spatial control over doping levels. Combined terahertz (THz) spectroscopy and electrical device measurements reveal that the Fermi level in laser-illuminated graphene can be controllably and reversibly tuned between p-and n-type in a large range (over ∼600 meV from −420 to +180 meV) by readily tuning the peak intensity and the duration of the laser irradiation treatment. Furthermore, we demonstrate that our photochemical approach for doping of graphene allows one to optically write doping structure with spatial control. Given the ease, effectiveness, and simplicity of the method, this photochemical doping mechanism offers a simple, reversible approach to control the steady-state electronic and optical properties of graphene.

Terahertz Nonlinear Optics of Graphene: From Saturable Absorption to High‐Harmonics Generation

“…For the typical doping levels for substrate‐supported graphene, resulting in a Fermi energy on the order of 100 meV (see, e.g., refs. ), the interband THz transitions are Pauli‐blocked even at T = 0, leaving only the intraband transitions described by the free carrier conductivity of Equation possible.…”

“…We note that most single‐layer graphene samples are in fact unintentionally doped due to impurities induced in the preparation process as well as the effects of the environment in which graphene is used . For example, in the course of preparation of chemical vapor deposition (CVD) grown graphene, which is one of the most popular graphene forms broadly available nowadays, hole‐doping is induced during the transfer of poly(methyl methacrylate) (PMMA)‐covered graphene from copper foils to dielectric substrates by residues of PMMA .…”

“…Integration of the density of states in graphene (Equation ) within 1 THz = 4.1 meV band around the Dirac point yields only about N ≈ 10 8 cm −2 electrons available for interband THz transitions. On the other hand, a typical doping of graphene readily yields a Fermi energy E F ≈ 100 meV and free carrier density of N c ≈ 10 12 cm −2 (see Equation ), available for the interaction with the THz fields via intraband conductivity, and leading to a typical THz power absorbance on the order of A intra ≈ 0.1 at room temperature (see, e.g., refs. ).…”

The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies

“…The ultrafast energy relaxation dynamics of graphene have been studied experimentally using optical pump -optical probe spectroscopy [21][22][23][24][25][26], time-resolved ARPES measurements [27,28], time-resolved photocurrent scanning microscopy [29][30][31], optical pump -terahertz (THz) probe spectroscopy [32][33][34][35][36][37][38][39][40][41][42][43][44] and high-field THz spectroscopy [6,45]. These time-resolved studies have identified that the energy relaxation dynamics of the photoexcited carriers consist mainly of carrier-carrier scattering and coupling to optical, acoustic and remote substrate phonons.…”

Dimensional Confinement in Carbon‐based Structures – From 3D to 1D

“…In contrast to such uncontrolled alterations of graphene's electronic properties, a promising approach to tailor the band structure and the charge carrier properties is doping by heteroatoms, which are either incorporated in the lattice replacing carbon atoms, or physisorbed to the graphene surface while leaving the lattice intact . Combining both approaches, allows for flexible transitions between n‐ and p‐type doping in the same crystal, which demonstrates the versatility of changing the electronic properties in graphene by controlling the degree of doping . Furthermore, heteroatomic doping has even the potential to open up a band gap .…”