| Summary: | Graphene oxides with different degrees of oxidation are prepared by controlling UV irradiation on graphene, and the charge transport and the evolution of the transport gap are investigated according to the extent of oxidation. With increasing oxygenous defect density <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>n</mi><mi mathvariant="normal">D</mi></msub></semantics></math></inline-formula>, a transition from ballistic to diffusive conduction occurs at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>n</mi><mi mathvariant="normal">D</mi></msub><mo>≃</mo><msup><mn>10</mn><mn>12</mn></msup></mrow></semantics></math></inline-formula> cm<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup></semantics></math></inline-formula> and the transport gap grows in proportion to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msqrt><msub><mi>n</mi><mi mathvariant="normal">D</mi></msub></msqrt></semantics></math></inline-formula>. Considering the potential fluctuation related to the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>e</mi><mo>−</mo><mi>h</mi></mrow></semantics></math></inline-formula> puddle, the bandgap of graphene oxide is deduced to be <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>E</mi><mi mathvariant="normal">g</mi></msub><mo>≃</mo><mn>30</mn><msqrt><mrow><msub><mi>n</mi><mi mathvariant="normal">D</mi></msub><mspace width="3.33333pt"></mspace><mrow><mo>(</mo><msup><mn>10</mn><mn>12</mn></msup><mspace width="3.33333pt"></mspace><msup><mi>cm</mi><mrow><mo>−</mo><mn>2</mn></mrow></msup><mo>)</mo></mrow></mrow></msqrt></mrow></semantics></math></inline-formula> meV. The temperature dependence of conductivity showed metal–insulator transitions at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>n</mi><mi mathvariant="normal">D</mi></msub><mo>≃</mo><mn>0.3</mn><mo>×</mo><msup><mn>10</mn><mn>12</mn></msup></mrow></semantics></math></inline-formula> cm<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup></semantics></math></inline-formula>, consistent with Ioffe–Regel criterion. For graphene oxides at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>n</mi><mi mathvariant="normal">D</mi></msub><mo>≥</mo><mn>4.9</mn><mo>×</mo><msup><mn>10</mn><mn>12</mn></msup></mrow></semantics></math></inline-formula> cm<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup></semantics></math></inline-formula>, analysis indicated charge transport occurred via 2D variable range hopping conduction between localized <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>s</mi><msup><mi>p</mi><mn>2</mn></msup></mrow></semantics></math></inline-formula> domain. Our work elucidates the transport mechanism at different extents of oxidation and supports the possibility of adjusting the bandgap with oxygen content.
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