Abstract

The chemical activity of ionized hydrogen describes the potency of protons in solution and is used to define the pH with wide-ranging applications in biological and materials sciences. Measuring pH with graphene field-effect transistors (FETs) has been described in the past but has yet to be characterized with a monolayer hexagonal boron nitride (hBN) capping layer. hBN capping is hypothesized to lower the chemical affinity of protons to oxidized defects in the graphene crystal and lower the standard deviation of pH measurement via screening charge density. First, the electronic properties of commercial, monolayer graphene and monolayer hBN/graphene, both on four-inch 90 nm SiO₂/p-type Si, were contrasted as a function of solutal pH in 10 mM phosphate buffered solution. The two-dimensional (2D) FETs were fabricated with photoresistless metallization followed by microcentrifuge tube-masking and reactive ion etching to define a quasi-pure 2D sensing mesa for pH sensing. Thereafter, the devices were exposed to phosphate buffer of varying pH and the resistance and liquid-transfer characteristics measured. The Dirac voltages of both devices became less positive with increasing pH. The hBN/graphene/SiO₂ FETs had higher linearity, and both lower standard deviation and range of Dirac voltage shifting than monolayer graphene/SiO2. Then, before microcentrifuge tube-masking, freshly metallized devices were coated with 9 nm of Al₂O₃ by electron beam deposition. The Al₂O₃/graphene/SiO₂ and Al₂O₃/hBN/graphene/SiO₂ devices showed an altered response to the pH of the phosphate buffered solutions before and after basic etching of the Al₂O₃ with NaOH (pH = 12). This prompted studying the pH-dependencies of the 2D devices as a function of the thickness of Al₂O₃ via atomic layer deposition. These results show that modifying the surface of monolayer graphene with nanoscale dielectrics (monolayer hBN and/or Al₂O₃) enables tuning of the pH-dependent electronic properties of graphene pH sensors.

Abstract

The chemical activity of ionized hydrogen describes the potency of protons in solution and is used to define the pH with wide-ranging applications in biological and materials sciences. Measuring pH with graphene field-effect transistors (FETs) has been described in the past but has yet to be characterized with a monolayer hexagonal boron nitride (hBN) capping layer. hBN capping is hypothesized to lower the chemical affinity of protons to oxidized defects in the graphene crystal and lower the standard deviation of pH measurement via screening charge density. First, the electronic properties of commercial, monolayer graphene and monolayer hBN/graphene, both on four-inch 90 nm SiO₂/p-type Si, were contrasted as a function of solutal pH in 10 mM phosphate buffered solution. The two-dimensional (2D) FETs were fabricated with photoresistless metallization followed by microcentrifuge tube-masking and reactive ion etching to define a quasi-pure 2D sensing mesa for pH sensing. Thereafter, the devices were exposed to phosphate buffer of varying pH and the resistance and liquid-transfer characteristics measured. The Dirac voltages of both devices became less positive with increasing pH. The hBN/graphene/SiO₂ FETs had higher linearity, and both lower standard deviation and range of Dirac voltage shifting than monolayer graphene/SiO2. Then, before microcentrifuge tube-masking, freshly metallized devices were coated with 9 nm of Al₂O₃ by electron beam deposition. The Al₂O₃/graphene/SiO₂ and Al₂O₃/hBN/graphene/SiO₂ devices showed an altered response to the pH of the phosphate buffered solutions before and after basic etching of the Al₂O₃ with NaOH (pH = 12). This prompted studying the pH-dependencies of the 2D devices as a function of the thickness of Al₂O₃ via atomic layer deposition. These results show that modifying the surface of monolayer graphene with nanoscale dielectrics (monolayer hBN and/or Al₂O₃) enables tuning of the pH-dependent electronic properties of graphene pH sensors.

Abstract

The chemical activity of ionized hydrogen describes the potency of protons in solution and is used to define the pH with wide-ranging applications in biological and materials sciences. Measuring pH with graphene field-effect transistors (FETs) has been described in the past but has yet to be characterized with a monolayer hexagonal boron nitride (hBN) capping layer. hBN capping is hypothesized to lower the chemical affinity of protons to oxidized defects in the graphene crystal and lower the standard deviation of pH measurement via screening charge density. First, the electronic properties of commercial, monolayer graphene and monolayer hBN/graphene, both on four-inch 90 nm SiO₂/p-type Si, were contrasted as a function of solutal pH in 10 mM phosphate buffered solution. The two-dimensional (2D) FETs were fabricated with photoresistless metallization followed by microcentrifuge tube-masking and reactive ion etching to define a quasi-pure 2D sensing mesa for pH sensing. Thereafter, the devices were exposed to phosphate buffer of varying pH and the resistance and liquid-transfer characteristics measured. The Dirac voltages of both devices became less positive with increasing pH. The hBN/graphene/SiO₂ FETs had higher linearity, and both lower standard deviation and range of Dirac voltage shifting than monolayer graphene/SiO2. Then, before microcentrifuge tube-masking, freshly metallized devices were coated with 9 nm of Al₂O₃ by electron beam deposition. The Al₂O₃/graphene/SiO₂ and Al₂O₃/hBN/graphene/SiO₂ devices showed an altered response to the pH of the phosphate buffered solutions before and after basic etching of the Al₂O₃ with NaOH (pH = 12). This prompted studying the pH-dependencies of the 2D devices as a function of the thickness of Al₂O₃ via atomic layer deposition. These results show that modifying the surface of monolayer graphene with nanoscale dielectrics (monolayer hBN and/or Al₂O₃) enables tuning of the pH-dependent electronic properties of graphene pH sensors.

Abstract

The chemical activity of ionized hydrogen describes the potency of protons in solution and is used to define the pH with wide-ranging applications in biological and materials sciences. Measuring pH with graphene field-effect transistors (FETs) has been described in the past but has yet to be characterized with a monolayer hexagonal boron nitride (hBN) capping layer. hBN capping is hypothesized to lower the chemical affinity of protons to oxidized defects in the graphene crystal and lower the standard deviation of pH measurement via screening charge density. First, the electronic properties of commercial, monolayer graphene and monolayer hBN/graphene, both on four-inch 90 nm SiO₂/p-type Si, were contrasted as a function of solutal pH in 10 mM phosphate buffered solution. The two-dimensional (2D) FETs were fabricated with photoresistless metallization followed by microcentrifuge tube-masking and reactive ion etching to define a quasi-pure 2D sensing mesa for pH sensing. Thereafter, the devices were exposed to phosphate buffer of varying pH and the resistance and liquid-transfer characteristics measured. The Dirac voltages of both devices became less positive with increasing pH. The hBN/graphene/SiO₂ FETs had higher linearity, and both lower standard deviation and range of Dirac voltage shifting than monolayer graphene/SiO2. Then, before microcentrifuge tube-masking, freshly metallized devices were coated with 9 nm of Al₂O₃ by electron beam deposition. The Al₂O₃/graphene/SiO₂ and Al₂O₃/hBN/graphene/SiO₂ devices showed an altered response to the pH of the phosphate buffered solutions before and after basic etching of the Al₂O₃ with NaOH (pH = 12). This prompted studying the pH-dependencies of the 2D devices as a function of the thickness of Al₂O₃ via atomic layer deposition. These results show that modifying the surface of monolayer graphene with nanoscale dielectrics (monolayer hBN and/or Al₂O₃) enables tuning of the pH-dependent electronic properties of graphene pH sensors.