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AlGaAs heterostructures
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Physica E 21 (2004) 1111 – 1115 www.elsevier.com/locate/physe
Liquid phase sensors based on chemically functionalized GaAs/AlGaAs heterostructures S.M. Lubera , K. Adlkoferb , U. Ranta , A. Ulmanc , A. G-olzh-auserd , M. Grunzed , D. Schuha , M. Tanakab , M. Tornowa;∗ , G. Abstreitera a Walter
Schottky Institut, Technische Universitat Munchen, Am Coulombwall, Garching 85748, Germany fur Biophysik E22, Technische Universitat Munchen, Garching 85748, Germany c Department of Chemical Engineering and Chemistry, Polytechnic University, New York 11201, USA d Lehrstuhl f ur Angewandte Physikalische Chemie, Universitat Heidelberg, Heidelberg 69120, Germany b Lehrstuhl
Abstract We report on surface-near two-dimensional electron gases in GaAs/AlGaAs heterostructures for application in potential chemical or biochemical sensors in liquid environment. GaAs cap layers of the heterostructures were coated with self-assembled monolayers of 4 -substituted 4-mercaptobiphenyls (MBP), showing stable device performance in physiological (aqueous) bu4ers. Deposition of MBP with di4erent 4 -substituents led to systematic changes in the lateral resistance, which can be correlated to the electrical potential drop across the established surface dipole layers. Furthermore, the lateral resistance showed a clear dependency on organic solvents with di4erent polarities, suggesting its high sensitivity to the polarity of physisorbed molecules. ? 2003 Elsevier B.V. All rights reserved. PACS: 81.07.Pr; 73.40.−c; 81.65.Rv; 07.07.Df Keywords: GaAs; Heterostructure; Self-assembled monolayer; Sensor; Two-dimensional electron gas
Gate-less semiconductor =eld e4ect transistors have a large potential as smart sensing devices for biochemical processes at surfaces [1], such as the label-free detection of, e.g., DNA hybridization [2], speci=c protein–antibody binding [3] or neural signaling [4,5]. Their advantages are the widely tunable electrical properties, their applicability to array-scale integration by miniaturization and high sensitivity to surface potential changes. However, the interface
∗ Corresponding author. Tel.: +49-89-289-12772; fax: +49-89320-6620. E-mail address: [email protected] (M. Tornow).
between semiconductor substrates and physiological electrolytes is a fundamental issue. Besides the ability to allow e4ective signal transfer from the surface to the sensor structure inside the substrate, it should be suitable for biochemical functionalization and obligatory, should passivate the surface against electrochemical decomposition [6]. In the case of III–V compound semiconductor systems such as GaAs/AlGaAs such functions can be convincingly combined by coating the surface with thin organic layers. The chemisorption of molecules modi=es the surface energetic [7,8] and electronic transport properties [9,10]. Previously, the electrochemical passivation of GaAs [1 0 0] surfaces by
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.189
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S.M. Luber et al. / Physica E 21 (2004) 1111 – 1115
deposition of self-assembled monolayers (SAM) of alkanethiols was reported [11] and successfully transferred to surface-near InAs quantum dot systems buried in an AlGaAs/GaAs heterostructure [12,13]. More recently, the stable coating of bulk GaAs with mercaptobiphenyl (MBP) monolayers was accomplished [14,15]. In this report, we present lateral resistance measurements on GaAs/AlGaAs heterostructures passivated with SAMs of MBP in aqueous bu4ers, different organic solvents, and in air. While a bare, un-passivated device rapidly degrades in the electrolyte, the monolayer-coated one shows good stability against electrochemical decomposition. Immersing the protected devices in various organic solvents leads to characteristic, reversible changes in resistance. This also con=rms the chemical stability of this monomolecular =lm, and suggests the device’s sensitivity to polar molecules on the surface. In fact, coating the devices by MBPs with di4erent 4 -substitutions leads to resistances linearly increasing with the molecules’ dipole moments. GaAs/AlGaAs heterostructures, containing a two-dimensional electron gas (2DEG) 60 nm beneath the surface, were grown by molecular beam epitaxy and patterned into Hallbar-like mesa structures by standard lithographic techniques. As references, on some of the samples TiAu Schottky gates were deposited. After the surface coating with MBP-SAMs (see Ref. [15] for the detailed procedure) the monolayers of some of the samples were additionally exposed to e-beam irradiation leading to a cross-linking of the phenyl rings, in order to increase the =lm’s chemical stability [16]. Lateral transport measurements (entirely in the dark) were either performed by biasing source to drain by a constant DC voltage (100 mV) and recording the resulting current (close to transistor channel pinch-o4), or in a 4-terminal set-up for resistances of a few kK (inset of Fig. 3). A peristaltic pump operated Low-chamber system restricted the exposure of liquids to the sample surface to the central sensing part of the device, not touching the outer metal contacts. Sample and Luid temperatures were controlled to 23◦ C. Fig. 1 shows the resistance of a device coated with cross-linked MBP in 0:01 M phosphate bu4ered saline solution (PBS) at pH = 6:5 as a function of
Fig. 1. Sample resistance vs. time for a bare GaAs/AlGaAs device (squares) and one coated with cross-linked MBP–SAM (circles) in 0:01 M phosphate bu4ered saline solution (PBS) at pH 6.50, ionic strength 0:1 M, T = 23◦ C. Inset: Schematic cross-section of the hybrid device. The label X at the MBP denotes a variable chemical endgroup (here X = H ).
time in comparison to an unfunctionalized sample. The bare sample rapidly degrades due to successive surface decomposition, whereas, the MBP-passivated sample showed good stability over several hours. The observed slight resistance increase can be attributed to defects remaining in the organic layer which are likely to occur at the mesa edges and will be e4ectively reduced in future miniaturization and planarization/encapsulation. As the next step, a device was functionalized with 4 -methyl-4-MBP (MBP-CH3 ) and subjected to a variety of organic solvents. Upon injection of isopropanol, the sample resistance quickly increased from about 70 kK to above 10 MK (Fig. 2) and saturated within about 10 min to ∼1 MK thereafter. Purging with N2 recovered the ambient value within the same order of magnitude. Di4erent solvents gave rise to analogous responses however with di4erent saturation values. Coatings with MBP having other endgroups than CH3 showed a qualitatively similar behaviour. Note that non-polar solvents like octane (or cyclohexane) did not induce any resistance change. In contrast to the SAM-modi=ed devices, barely GaAs-capped ones showed much less reproducible and unstable results (data not shown). Besides that these results suggest a good stability against organic solvents, the SAM coating apparently also
S.M. Luber et al. / Physica E 21 (2004) 1111 – 1115
Fig. 2. E4ect of di4erent solvents on the resistance of a sample coated with MBP-CH3 . After initial rinsing, the sample was kept in each solvent for several minutes until the resistance value saturated. After a subsequent brief rinsing with the same liquid the surface was dry purged with N2 . Note that no resistance change was observed for non-polar octane.
functionalizes the FET as a chemical sensing device for polar solvents, similar to a previous report on GaN devices [17]. To gain more insight in the underlying electrostatics we measured devices coated with MBP, MBP-CH3 , and MBP-OH (4 -hydroxyl-4-MBP) in ambient air. Here conjugation between 4 -substitutents and thiolate can introduce di4erent molecular dipole moments (DM) but note that all MBPs were covalently bound to the substrate by the same S–As bond. In Fig. 3 the measured square resistances are plotted as a function of the projection of the individual DMs along the molecule’s long axis. 1 Cleary, the sheet resistance of the surface-near 2DEG increases monotonically. The functionalized GaAs/AlGaAs heterostructure device is operated as a =eld e4ect transistor (FET) whose 2D sheet resistance is controlled by the surface potential via the band bending (BB). The adsorption of a dipole layer results in a change of the electron 1 The MBP DM values and electron densities were estimated from DFT calculations for individual (thiol-terminated) MBP molecules, with CACheJ software using the DZVP basis-set at the GGA-B88LYP level.
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Fig. 3. Mean square resistances (in air) for samples covered with MBP of three di4erently substituted chemical endgroups in comparison to a bare sample, as a function of the projected molecular DM. Inset: Schematic 4-terminal set-up.
work function given by the sum of changes in electron aQnity (EA) and surface BB, R = R + RVbb . Here, the variation of EA is given by the potential drop through the layer of adsorbed molecules. In the simplest approximation (ideal parallel plate capacitor (PPC)) one obtains by elementary electrostatics for the potential drop the Helmholtz equation RVdip = np cos =0
(1)
with n being the areal dipole density, p the absolute DM, the dipole vector’s tilt angle vs. surface normal and the layer’s dielectric constant. In general, both cannot be identi=ed with its bulk macroscopic [18] and p not with its free molecule’s [19] value. Nevertheless, a linear dependence of R from p as in Eq. (1) has been observed [20]. However, a change of the BB due to dipole adsorption is less evident, as such an ideal PPC has no external electrical =eld. In fact, the MBP monolayer might deviate from an ideal PPC: =rst, the underlying presumption that the dipole “length” w exceeds the lateral dipole spacing a is only approximately ful=lled, U (layer thickness) [15] and taking w 6 d = 10 ± 2 A U from n = 0:031 A U −2 (density) [21]. Second, a = 5:6 A formation of the chemical bond likely modi=es the surface states distribution and thus the surface Fermi level pinning. However, simulations showed that to a =rst approximation this contribution should not
1114
S.M. Luber et al. / Physica E 21 (2004) 1111 – 1115
Fig. 4. Saturation values of the resistance of the MBP-CH3 device from Fig. 2, after exponentially =tting the resistance decays over time, as a function of p= (see footnote 2). The line is a guide to the eye.
distinguish between the di4erently terminated MBPs (See footnote 1). Additional deviations can be due to local defects in the layer or at the Hall bar edges. The potential drop through the dipole monolayer may only “convert” totally to a change in BB if the electrostatic potential is kept =xed above it, e.g., in an ionic solution held on a reference potential [9,22]. Although the electrostatic boundary condition cannot be de=ned in our experimental set-up that way, we =nd an obvious correlation between RVdip and RVbb . For the data shown in Fig. 3 we estimate potential drops according to Eq. (1): RVdip; H = 160 mV, RVdip; OH = 200 mV and RVdip; CH3 =280 mV by assuming r =4:5 [23] and a mean =30◦ [14]. The order of magnitude of the resistance changes =ts very well to independent reference measurements (sample with Schottky gate), where the same relative resistance changes were observed for RVGate =90, 120 and 140 mV, respectively. The semiconductor-SAM-solvent system was empirically analyzed in a similar way. Fig. 4 displays the saturation resistances (from Fig. 2) plotted as a function of p= for each solvent. Again, we obtain a roughly linear resistance increase. Reference resistances in the same range were obtained at gate voltages 2 Numerical values for p and (bulk value) from Beilstein database: Octane (p=0 D; =1:95), Methanol (1.7, 32.8), Ethanol (1.68, 24.3), 1-Pentanol (1.65, 14.3), Acetone (2.88, 20.6), Isopropanol (1.66, 18.8), Acetonitrile (3.92, 36.9), N2 (0, 1).
beyond −200 mV. It is interesting to note that these surface potential values would correspond to the sum of the voltage drop RVdip of the MBP-CH3 layer and that across a =rst layer of adsorbed solvent molecules (after Eq. (1)). Here we assumed monomolecular adsorption of solvent molecules (“bimolecular” monolayer) [22,24], whose DM point out in the same direction as the MBP molecules. In spite of the unknown orientation of solvent molecules on SAM surfaces, this crude approximation seems to agree with the obtained resistance. In summary, we reported on electronic transport measurements on 2DEGs in AlGaAs/GaAs heterostructure devices, which have been passivated by an MBP-SAM. The device showed a good stability against electrochemical decomposition in aqueous bu4ers. Di4erent 4 -substituents allowed for a control of molecular DM resulting in a change in surface potential which agreed well with the estimated electrostatic potential drop through the monolayer. Furthermore, the resistance in the presence of different organic solvents showed a systematic change. Although, a quantitative explanation of the observed e4ects cannot be given at present, reference measurements suggest a correlation between the voltage drop through “bimolecular” monolayers and the change in surface potential. The demonstrated sensitivity and stability propose a potential of the hybrid device for future chemical and biological sensing applications. The authors thank A. Paul for e-beam cross-linking of some of the MBP samples and M.G. Nikolaides for help with the experimental set-up. This work was =nancially supported by the DFG (SFB563, Ta 259/1) and in part by Fujitsu Labs Europe. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
P. Bergveld, Sens. Actuators B 88 (2003) 1. J. Fritz, et al., Proc. Natl. Acad. Sci. 99 (2002) 14142. Y. Cui, et al., Science 293 (2001) 1289. P. Fromherz, Chem. Phys. Chem. 3 (2002) 276. A. O4enh-ausser, W. Knoll, TIBTECH 19 (2001) 62. E. Sackmann, M. Tanaka, TIBTECH 18 (2000) 58. F. Seker, et al., Chem. Rev. 100 (2000) 2505. A. Vilan, D. Cahen, TIBTECH 20 (2002) 22. D.G. Wu, et al., Chem. Eur. J. 7 (2001) 1743. K. Gartsmam, et al., Chem. Phys. Lett. 283 (1998) 301. K. Adlkofer, et al., Appl. Phys. Lett. 76 (2000) 3313. E.F. Duijs, et al., Phys. Stat. Sol. B 224 (2001) 871. K. Adlkofer, et al., Phys. Chem. Chem. Phys. 4 (2002) 785.
S.M. Luber et al. / Physica E 21 (2004) 1111 – 1115 [14] [15] [16] [17] [18]
A. Shaporenko, et al., Langmuir 19 (2003) 4992. K. Adlkofer, et al., J. Phys. Chem. B 107 (2003) 587. W. Geyer, et al., Appl. Phys. Lett. 75 (1999) 2401. R. Neuberger, et al., Phys. Stat. Sol. A 185 (2001) 85. H. L-uth, Surfaces and Interfaces of Solid Materials, Springer, Berlin, 1997.
[19] [20] [21] [22] [23] [24]
1115
D.M. Taylor, G.F. Bayes, Phys. Rev. E 49 (1994) 1439. S. Bastide, et al., J. Phys. Chem. B 101 (1997) 2678. C.D. Bain, Adv. Mater. 4 (1992) 591. L. Chai, D. Cahen, Mat. Sci. Eng. 19 (2002) 339. E. Sabatani, et al., Langmuir 9 (1993) 2974. G.R. Hutchison, et al., J. Phys. Chem. B 105 (2001) 2881.
Physica E 21 (2004) 1111 – 1115 www.elsevier.com/locate/physe
Liquid phase sensors based on chemically functionalized GaAs/AlGaAs heterostructures S.M. Lubera , K. Adlkoferb , U. Ranta , A. Ulmanc , A. G-olzh-auserd , M. Grunzed , D. Schuha , M. Tanakab , M. Tornowa;∗ , G. Abstreitera a Walter
Schottky Institut, Technische Universitat Munchen, Am Coulombwall, Garching 85748, Germany fur Biophysik E22, Technische Universitat Munchen, Garching 85748, Germany c Department of Chemical Engineering and Chemistry, Polytechnic University, New York 11201, USA d Lehrstuhl f ur Angewandte Physikalische Chemie, Universitat Heidelberg, Heidelberg 69120, Germany b Lehrstuhl
Abstract We report on surface-near two-dimensional electron gases in GaAs/AlGaAs heterostructures for application in potential chemical or biochemical sensors in liquid environment. GaAs cap layers of the heterostructures were coated with self-assembled monolayers of 4 -substituted 4-mercaptobiphenyls (MBP), showing stable device performance in physiological (aqueous) bu4ers. Deposition of MBP with di4erent 4 -substituents led to systematic changes in the lateral resistance, which can be correlated to the electrical potential drop across the established surface dipole layers. Furthermore, the lateral resistance showed a clear dependency on organic solvents with di4erent polarities, suggesting its high sensitivity to the polarity of physisorbed molecules. ? 2003 Elsevier B.V. All rights reserved. PACS: 81.07.Pr; 73.40.−c; 81.65.Rv; 07.07.Df Keywords: GaAs; Heterostructure; Self-assembled monolayer; Sensor; Two-dimensional electron gas
Gate-less semiconductor =eld e4ect transistors have a large potential as smart sensing devices for biochemical processes at surfaces [1], such as the label-free detection of, e.g., DNA hybridization [2], speci=c protein–antibody binding [3] or neural signaling [4,5]. Their advantages are the widely tunable electrical properties, their applicability to array-scale integration by miniaturization and high sensitivity to surface potential changes. However, the interface
∗ Corresponding author. Tel.: +49-89-289-12772; fax: +49-89320-6620. E-mail address: [email protected] (M. Tornow).
between semiconductor substrates and physiological electrolytes is a fundamental issue. Besides the ability to allow e4ective signal transfer from the surface to the sensor structure inside the substrate, it should be suitable for biochemical functionalization and obligatory, should passivate the surface against electrochemical decomposition [6]. In the case of III–V compound semiconductor systems such as GaAs/AlGaAs such functions can be convincingly combined by coating the surface with thin organic layers. The chemisorption of molecules modi=es the surface energetic [7,8] and electronic transport properties [9,10]. Previously, the electrochemical passivation of GaAs [1 0 0] surfaces by
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.189
1112
S.M. Luber et al. / Physica E 21 (2004) 1111 – 1115
deposition of self-assembled monolayers (SAM) of alkanethiols was reported [11] and successfully transferred to surface-near InAs quantum dot systems buried in an AlGaAs/GaAs heterostructure [12,13]. More recently, the stable coating of bulk GaAs with mercaptobiphenyl (MBP) monolayers was accomplished [14,15]. In this report, we present lateral resistance measurements on GaAs/AlGaAs heterostructures passivated with SAMs of MBP in aqueous bu4ers, different organic solvents, and in air. While a bare, un-passivated device rapidly degrades in the electrolyte, the monolayer-coated one shows good stability against electrochemical decomposition. Immersing the protected devices in various organic solvents leads to characteristic, reversible changes in resistance. This also con=rms the chemical stability of this monomolecular =lm, and suggests the device’s sensitivity to polar molecules on the surface. In fact, coating the devices by MBPs with di4erent 4 -substitutions leads to resistances linearly increasing with the molecules’ dipole moments. GaAs/AlGaAs heterostructures, containing a two-dimensional electron gas (2DEG) 60 nm beneath the surface, were grown by molecular beam epitaxy and patterned into Hallbar-like mesa structures by standard lithographic techniques. As references, on some of the samples TiAu Schottky gates were deposited. After the surface coating with MBP-SAMs (see Ref. [15] for the detailed procedure) the monolayers of some of the samples were additionally exposed to e-beam irradiation leading to a cross-linking of the phenyl rings, in order to increase the =lm’s chemical stability [16]. Lateral transport measurements (entirely in the dark) were either performed by biasing source to drain by a constant DC voltage (100 mV) and recording the resulting current (close to transistor channel pinch-o4), or in a 4-terminal set-up for resistances of a few kK (inset of Fig. 3). A peristaltic pump operated Low-chamber system restricted the exposure of liquids to the sample surface to the central sensing part of the device, not touching the outer metal contacts. Sample and Luid temperatures were controlled to 23◦ C. Fig. 1 shows the resistance of a device coated with cross-linked MBP in 0:01 M phosphate bu4ered saline solution (PBS) at pH = 6:5 as a function of
Fig. 1. Sample resistance vs. time for a bare GaAs/AlGaAs device (squares) and one coated with cross-linked MBP–SAM (circles) in 0:01 M phosphate bu4ered saline solution (PBS) at pH 6.50, ionic strength 0:1 M, T = 23◦ C. Inset: Schematic cross-section of the hybrid device. The label X at the MBP denotes a variable chemical endgroup (here X = H ).
time in comparison to an unfunctionalized sample. The bare sample rapidly degrades due to successive surface decomposition, whereas, the MBP-passivated sample showed good stability over several hours. The observed slight resistance increase can be attributed to defects remaining in the organic layer which are likely to occur at the mesa edges and will be e4ectively reduced in future miniaturization and planarization/encapsulation. As the next step, a device was functionalized with 4 -methyl-4-MBP (MBP-CH3 ) and subjected to a variety of organic solvents. Upon injection of isopropanol, the sample resistance quickly increased from about 70 kK to above 10 MK (Fig. 2) and saturated within about 10 min to ∼1 MK thereafter. Purging with N2 recovered the ambient value within the same order of magnitude. Di4erent solvents gave rise to analogous responses however with di4erent saturation values. Coatings with MBP having other endgroups than CH3 showed a qualitatively similar behaviour. Note that non-polar solvents like octane (or cyclohexane) did not induce any resistance change. In contrast to the SAM-modi=ed devices, barely GaAs-capped ones showed much less reproducible and unstable results (data not shown). Besides that these results suggest a good stability against organic solvents, the SAM coating apparently also
S.M. Luber et al. / Physica E 21 (2004) 1111 – 1115
Fig. 2. E4ect of di4erent solvents on the resistance of a sample coated with MBP-CH3 . After initial rinsing, the sample was kept in each solvent for several minutes until the resistance value saturated. After a subsequent brief rinsing with the same liquid the surface was dry purged with N2 . Note that no resistance change was observed for non-polar octane.
functionalizes the FET as a chemical sensing device for polar solvents, similar to a previous report on GaN devices [17]. To gain more insight in the underlying electrostatics we measured devices coated with MBP, MBP-CH3 , and MBP-OH (4 -hydroxyl-4-MBP) in ambient air. Here conjugation between 4 -substitutents and thiolate can introduce di4erent molecular dipole moments (DM) but note that all MBPs were covalently bound to the substrate by the same S–As bond. In Fig. 3 the measured square resistances are plotted as a function of the projection of the individual DMs along the molecule’s long axis. 1 Cleary, the sheet resistance of the surface-near 2DEG increases monotonically. The functionalized GaAs/AlGaAs heterostructure device is operated as a =eld e4ect transistor (FET) whose 2D sheet resistance is controlled by the surface potential via the band bending (BB). The adsorption of a dipole layer results in a change of the electron 1 The MBP DM values and electron densities were estimated from DFT calculations for individual (thiol-terminated) MBP molecules, with CACheJ software using the DZVP basis-set at the GGA-B88LYP level.
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Fig. 3. Mean square resistances (in air) for samples covered with MBP of three di4erently substituted chemical endgroups in comparison to a bare sample, as a function of the projected molecular DM. Inset: Schematic 4-terminal set-up.
work function given by the sum of changes in electron aQnity (EA) and surface BB, R = R + RVbb . Here, the variation of EA is given by the potential drop through the layer of adsorbed molecules. In the simplest approximation (ideal parallel plate capacitor (PPC)) one obtains by elementary electrostatics for the potential drop the Helmholtz equation RVdip = np cos =0
(1)
with n being the areal dipole density, p the absolute DM, the dipole vector’s tilt angle vs. surface normal and the layer’s dielectric constant. In general, both cannot be identi=ed with its bulk macroscopic [18] and p not with its free molecule’s [19] value. Nevertheless, a linear dependence of R from p as in Eq. (1) has been observed [20]. However, a change of the BB due to dipole adsorption is less evident, as such an ideal PPC has no external electrical =eld. In fact, the MBP monolayer might deviate from an ideal PPC: =rst, the underlying presumption that the dipole “length” w exceeds the lateral dipole spacing a is only approximately ful=lled, U (layer thickness) [15] and taking w 6 d = 10 ± 2 A U from n = 0:031 A U −2 (density) [21]. Second, a = 5:6 A formation of the chemical bond likely modi=es the surface states distribution and thus the surface Fermi level pinning. However, simulations showed that to a =rst approximation this contribution should not
1114
S.M. Luber et al. / Physica E 21 (2004) 1111 – 1115
Fig. 4. Saturation values of the resistance of the MBP-CH3 device from Fig. 2, after exponentially =tting the resistance decays over time, as a function of p= (see footnote 2). The line is a guide to the eye.
distinguish between the di4erently terminated MBPs (See footnote 1). Additional deviations can be due to local defects in the layer or at the Hall bar edges. The potential drop through the dipole monolayer may only “convert” totally to a change in BB if the electrostatic potential is kept =xed above it, e.g., in an ionic solution held on a reference potential [9,22]. Although the electrostatic boundary condition cannot be de=ned in our experimental set-up that way, we =nd an obvious correlation between RVdip and RVbb . For the data shown in Fig. 3 we estimate potential drops according to Eq. (1): RVdip; H = 160 mV, RVdip; OH = 200 mV and RVdip; CH3 =280 mV by assuming r =4:5 [23] and a mean =30◦ [14]. The order of magnitude of the resistance changes =ts very well to independent reference measurements (sample with Schottky gate), where the same relative resistance changes were observed for RVGate =90, 120 and 140 mV, respectively. The semiconductor-SAM-solvent system was empirically analyzed in a similar way. Fig. 4 displays the saturation resistances (from Fig. 2) plotted as a function of p= for each solvent. Again, we obtain a roughly linear resistance increase. Reference resistances in the same range were obtained at gate voltages 2 Numerical values for p and (bulk value) from Beilstein database: Octane (p=0 D; =1:95), Methanol (1.7, 32.8), Ethanol (1.68, 24.3), 1-Pentanol (1.65, 14.3), Acetone (2.88, 20.6), Isopropanol (1.66, 18.8), Acetonitrile (3.92, 36.9), N2 (0, 1).
beyond −200 mV. It is interesting to note that these surface potential values would correspond to the sum of the voltage drop RVdip of the MBP-CH3 layer and that across a =rst layer of adsorbed solvent molecules (after Eq. (1)). Here we assumed monomolecular adsorption of solvent molecules (“bimolecular” monolayer) [22,24], whose DM point out in the same direction as the MBP molecules. In spite of the unknown orientation of solvent molecules on SAM surfaces, this crude approximation seems to agree with the obtained resistance. In summary, we reported on electronic transport measurements on 2DEGs in AlGaAs/GaAs heterostructure devices, which have been passivated by an MBP-SAM. The device showed a good stability against electrochemical decomposition in aqueous bu4ers. Di4erent 4 -substituents allowed for a control of molecular DM resulting in a change in surface potential which agreed well with the estimated electrostatic potential drop through the monolayer. Furthermore, the resistance in the presence of different organic solvents showed a systematic change. Although, a quantitative explanation of the observed e4ects cannot be given at present, reference measurements suggest a correlation between the voltage drop through “bimolecular” monolayers and the change in surface potential. The demonstrated sensitivity and stability propose a potential of the hybrid device for future chemical and biological sensing applications. The authors thank A. Paul for e-beam cross-linking of some of the MBP samples and M.G. Nikolaides for help with the experimental set-up. This work was =nancially supported by the DFG (SFB563, Ta 259/1) and in part by Fujitsu Labs Europe. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
P. Bergveld, Sens. Actuators B 88 (2003) 1. J. Fritz, et al., Proc. Natl. Acad. Sci. 99 (2002) 14142. Y. Cui, et al., Science 293 (2001) 1289. P. Fromherz, Chem. Phys. Chem. 3 (2002) 276. A. O4enh-ausser, W. Knoll, TIBTECH 19 (2001) 62. E. Sackmann, M. Tanaka, TIBTECH 18 (2000) 58. F. Seker, et al., Chem. Rev. 100 (2000) 2505. A. Vilan, D. Cahen, TIBTECH 20 (2002) 22. D.G. Wu, et al., Chem. Eur. J. 7 (2001) 1743. K. Gartsmam, et al., Chem. Phys. Lett. 283 (1998) 301. K. Adlkofer, et al., Appl. Phys. Lett. 76 (2000) 3313. E.F. Duijs, et al., Phys. Stat. Sol. B 224 (2001) 871. K. Adlkofer, et al., Phys. Chem. Chem. Phys. 4 (2002) 785.
S.M. Luber et al. / Physica E 21 (2004) 1111 – 1115 [14] [15] [16] [17] [18]
A. Shaporenko, et al., Langmuir 19 (2003) 4992. K. Adlkofer, et al., J. Phys. Chem. B 107 (2003) 587. W. Geyer, et al., Appl. Phys. Lett. 75 (1999) 2401. R. Neuberger, et al., Phys. Stat. Sol. A 185 (2001) 85. H. L-uth, Surfaces and Interfaces of Solid Materials, Springer, Berlin, 1997.
[19] [20] [21] [22] [23] [24]
1115
D.M. Taylor, G.F. Bayes, Phys. Rev. E 49 (1994) 1439. S. Bastide, et al., J. Phys. Chem. B 101 (1997) 2678. C.D. Bain, Adv. Mater. 4 (1992) 591. L. Chai, D. Cahen, Mat. Sci. Eng. 19 (2002) 339. E. Sabatani, et al., Langmuir 9 (1993) 2974. G.R. Hutchison, et al., J. Phys. Chem. B 105 (2001) 2881.