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On Monolayer Formation of Pyrenebutyric Acid on Graphene Malkolm Hinnemo, Jie Zhao, Patrik Ahlberg, Carl Hägglund, Viktor Djurberg, Ralph H. Scheicher, Shi-Li Zhang, and Zhibin Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04237 • Publication Date (Web): 28 Mar 2017 Downloaded from http://pubs.acs.org on April 2, 2017
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Figure 2. a) Ratio of sp2-sp2 carbon in PBA to sp2-sp2 carbon in graphene for different PBA concentrations fitted by equation (2), b) D’ peak fitted with K fixed to the values achieved in a), c) simulated energetically favorable morphology with PBA flat on graphene, and d) simulated energetically favorable morphology with 3 PBA binding to each other.
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Figure 3. a) Effective thickness of adsorbed PBA molecules determined by SE. The curve represents the fit with equation (2) by keeping K fixed to the values achieved in Figure 1a, and b) contact angle of a water drop for samples treated with different concentrations of PBA.
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Figure 1. a) Schematic of a PBA molecule, b) example of peak fit used in G and D’ peak analysis, and c) Raman spectra of SLG and SLG treated with 741 mM of PBA where the shaded area is used for peak deconvolution in (b). 1757x691mm (120 x 120 DPI)
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On Monolayer Formation of Pyrenebutyric Acid on Graphene Malkolm Hinnemo, 1 Jie Zhao, 1 Patrik Ahlberg, 1 Carl Hägglund, 1 Viktor Djurberg, 2 Ralph H. Scheicher, 2 Shi-Li Zhang, 1 Zhi-Bin Zhang1*
1
Division of Solid State Electronics, Department of Engineering Sciences, Uppsala University, SE-751 21 Uppsala, Sweden
2
Division of Materials Theory, Department of Physics and Astronomy, Uppsala University, SE-751 20 Uppsala, Sweden
ABSTRACT As a two-dimensional material with high charge carrier mobility, graphene may offer ultrahigh sensitivity in bio-sensing. In order to realize this, the first step is to functionalize the graphene. This is commonly done by using 1-pyrenebutyric acid (PBA) as a linker for biomolecules. However, the adsorption of PBA on graphene remains poorly understood despite reports of successful biosensors functionalized via this route. Here, the PBA adsorption on graphene is characterized through a combination of Raman spectroscopy, ab initio calculations, and spectroscopic ellipsometry. The PBA molecules are found to form a self-assembled monolayer on graphene, the formation of which is selflimiting and Langmuirian. Intriguingly, in concentrated solutions, the PBA molecules are found to stand up and stack horizontally with their edges contacting the graphene surface. This morphology could facilitate a surface densely populated with carboxylic functional groups. Spectroscopic analyses show ACS Paragon Plus Environment
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that the monolayer saturates at 5.3 PBA molecules per nm2 and measures ∼0.7 nm in thickness. The morphology study of this PBA monolayer sheds light on π-π stacking of small-molecule systems on graphene and provides an excellent base for optimizing functionalization procedures.
INTRODUCTION Graphene-based biosensors have received much attention in recent years due to their promise of an increase in sensitivity.1–3 Graphene is a hexagonal web of sp2-sp2 hybridized carbon atoms.4 As such, it has a high density of delocalized π-electrons that can interact with molecules. Lacking a bulk and having a low electrochemical reactivity on the basal planes while having a high reactivity on the edges makes graphene a highly attractive sensor material. A key step in sensor production is surface functionalization, a step that can be achieved in a multitude of ways. A common approach is to covalently attach functional groups to graphene.5 This, however, has the disadvantage of requiring defects or edges in graphene to allow for the attachment of the functional groups. It can thus not be used for applications where high conductivity or an efficient diffusion barrier is a requirement. For such applications, non-covalent approaches are desired. One such approach, which also is the most common one, is π-π stacking. A molecule that is often used for this type of functionalization is 1-pyrene butyric acid (PBA).6 As shown in Figure 1a, PBA consists of a pyrene group that contains π-electrons and a carboxylic group that can be used to facilitate further functionalization. Functionalization with this molecule on graphene flakes and carbon nanotubes in liquid solutions is well studied,7–10 but less is known on continuous graphene films that in turn are deposited on a substrate. Functionalization of graphene films obtained by chemical vapor deposition (CVD) can be of practical interest for their potential in large-scale sensor production and sensor array applications. Indeed, PBA-functionalized single-layer graphene (SLG) has been used in many types of sensors11 both for binding analytes to the graphene surface, and for changing its physical and chemical properties.12,13 However, the nature of the PBA binding on graphene is poorly understood. ACS Paragon Plus Environment
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Pyrene derivatives are used in functionalization of graphene because pyrene is aromatic and can form π-π stacking with graphene. It is then often anticipated that this interaction is face-to-face with an offset.14 However, this is not necessarily the case. A T-shaped binding of two benzene rings is known to have approximately the same binding energy as that of an offset face-to-face binding.15 Measurements on solid benzene has indeed shown the T-shaped binding to be the most common one in interaction between small aromatic molecules.16 This is ascribed to aromatic systems having a quadrupole moment with more negative charge in the center of the rings and a more positive one around the C atoms.17 Similar behavior is seen in other small aromatic molecules including pentacene, perflouropentacene, and diindenoperylene that form fishbone-like crystal structures.18 For the case of 3 benzene molecules, a triangular shape has been calculated to be the most stable.19 Pentacene, a molecule consisting of 5 fused benzene rings in a row, has been previously reported to bind at an angle to a graphene surface through arranging itself in a crystalline structure.20 When more molecules are involved, the situation gets more complicated and it becomes increasingly difficult to determine how the final structure could be. A computational study on adsorption of substituted benzene on graphene found that carboxylic groups exhibit stronger adhesion than the aromatic part of the molecule.21 This particularity could affect the binding of PBA and cause the carboxylic groups to be facing down. This study21 does, however, not take molecule-to-molecule interaction into effect and the bond strength between two carboxylic groups is of the same order of magnitude as that between the molecules and graphene making alternative configurations possible.22 An experimental study on similar molecules adsorbing on graphite23 found island formation which shows the significance of inter-molecular interaction. In order to continue improving the bio-sensing performance, an in-depth understanding of functionalization with derivatives of polyaromatic hydrocarbons is crucial. In the present work, we study the adsorption of PBA molecules on a graphene surface deposited on oxidized Si. As mentioned above, PBA is commonly used as a linker to anchor receptors onto graphene. The understanding of PBA adsorption in this context is of great importance but remains unknown. Aided with atomistic modeling based on density functional theory (DFT) calculations, we employ Raman spectroscopy in combination ACS Paragon Plus Environment
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with spectroscopic ellipsometry (SE) to investigate how PBA molecules interact with a CVD-grown SLG and reveal how the PBA molecules stand up and form a self-limiting monolayer on silicon oxide supported graphene at high PBA concentrations in inorganic solvents. METHOD The SLG was grown and transferred onto a 300 nm thick SiO2 layer on Si substrate in accordance with our previously reported procedure.24 Pyrene was deposited onto the transferred graphene film by immersion. A droplet of 100 µL of dimethylformamide (Sigma Aldrich) with varying concentrations of PBA (Sigma Aldrich) was cast atop the SLG covering the entire substrate. The solution was kept on the SLG for one hour under ambient settings. The samples with PBA-coated SLG were washed one time in DMF, two times in ethanol, and one time in isopropanol to remove superfluous PBA. In order to characterize the PBA coverage, Raman scattering was measured on all the samples on a Renishaw “inVia” Raman Spectrometer with a 20x lens. A 532 nm laser was used. Spectra of 18 spots were taken on each sample. The peaks were fitted using a non-linear optimization algorithm25 implemented in Matlab. The result of such a fit is shown in Figure 1b. The Si-peak at 520 cm-1 was used as reference for calibration as its intensity and position are not affected by the PBA deposition. All peak intensities were normalized to that of the Si-peak in order to account for variations among the measurements. For the Langmuir isotherm, the fit was carried out using the least square method with two fitting variables for the first G-peak regression and one variable for the following regressions. Contact angle measurements were made using a DinoLite AM7013MT digital microscope. Droplets of around 10 µL were placed on the sample surface with a pipette and the advancing and receding angles were measured. Atomistic computer modeling was employed to compare the energetics of two different stacking conformations of PBA molecules on SLG. To this end, DFT26,27 based simulations as implemented in SIESTA28 were carried out. Electrostatic interactions, such as hydrogen bonding, are readily taken into account by DFT calculations, but to properly include weak dispersive interactions, it was necessary to employ van der Waals correction29,30 to the generalized gradient approximation (PBE-GGA)31 for the ACS Paragon Plus Environment
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exchange-correlation functional in DFT along with norm-conserving pseudopotentials.32 Geometry relaxations were carried out until the forces on each atom crossed below the threshold of 0.1 eV/nm. SE was performed using a Woollam VASE system. The ellipsometric data (psi and delta) were recorded over wavelengths from 300 to 1000 nm for angles of incidence of 65, 70 and 75°. In the analysis, the samples were modeled as a stack of isotropic, homogeneous and uniform thickness layers. On top of the Si substrate, the layers were approximately 300 nm of SiO2 followed by SLG and PBA. The Si substrate was assumed to be optically thick and tabulated values from Woollam were used for its optical constants. The additional layers of the stack were characterized sequentially. First, measurements on a wafer with only SiO2 on Si were performed to extract the refractive index of the (non-absorbing) SiO2. The three parameters of a single Cauchy function along with the SiO2 thickness were used to fit the the model output by minimizing the root mean squared deviation from the measured data. The resulting dispersion for the refractive index was () = 1.453 + 3060/ + 3.43 × 10 / with the wavelength () in nanometers. The refractive index of the SiO2 was kept fixed in the further analysis. Next, SLG was deposited on samples cut from the analyzed wafer, and ellipsometric measurements were performed again. The permittivity of the SLG was modeled using a sum of one Lorentz and one Drude oscillator plus two poles. In order to enhance the robustness of the model fit, a multi-sample analysis was performed such that the data for different samples were simultaneously matched by means of the same parameter values for the optical constants of the graphene. The Drude term, on the form = −ℏ / ( + ℏ), had two adjustable parameters fitted to = 5.1 × 10 Ω-cm and = 0.172 fs. Here ℏ, and E are the Planck constant, vacuum permittivity and photon energy, respectively. The Lorentz oscillator, on the form = ! # $/(# − − $), had three fitted parameters ! = 13.6, # = 5.6 eV, and $ = 4.0 eV, respectively. The poles, on the form % = !/(# − ), were located outside the measured interval with parameters ! = −1.8 and 4.9 eV2, and # = 4.2 and 0 eV, respectively. The thicknesses of the SLG and SiO2 were fitted along with the optical constants of the SLG. Although the individual oscillator terms should not be given significant physical meaning here, the procedure ensured consistency with the Kramers-Kronig relation. Further, the ACS Paragon Plus Environment
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resulting sum gave a refractive index between 2.8 and 3.2 and an extinction coefficient between 1.4 and 1.6 in the visible range, which is quite consistent with literature values.33 Furthermore, the thickness of the SLG was fitted to 0.3 nm, which is close to the expected value. Finally, PBA was deposited on the SLG using varying solution concentrations, and the samples were measured once again. Again, a Kramers-Kronig consistent multi-sample analysis was performed, but in this case only the optical constants and (effective) thicknesses of the PBA layers and the SiO2 were allowed to vary between samples. Thus the SLG thickness and properties were now fixed, as was the properties of the SiO2 and Si. The resulting SiO2 thickness varied only slightly (±0.5 nm) between samples. The PBA layer was modeled as a sum of one Lorentz and one Cody-Lorentz34 oscillator, plus two poles. In total 11 parameters were used to fit the optical constants of 5 samples (produced using 0 to 1000 mM PBA solutions) over the specified wavelength range. The resulting refractive index was around 2, which is quite typical for organic materials. The extinction coefficient was zero for wavelengths exceeding 600 nm and slightly absorbing for shorter wavelengths. The main outcome of this analysis was the effective PBA thickness as a function of solution concentration.
Figure 1. a) Schematic of a PBA molecule, b) example of peak fit used in G and D’ peak analysis, and c) Raman spectra of SLG and SLG treated with 741 mM of PBA where the shaded area is used for peak deconvolution in (b).
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RESULTS AND DISCUSSION The Raman spectra of as-made SLG and SLG treated with 741 mM of PBA between 1100 and 3000 cm-1 are shown in Figure 1c. In this region of interest, the SLG shows six peaks: the D-peak at 1360, Gpeak at 1594, D’-peak at ~1620, N2-peak at 2350, D+D’’-peak at 2480, and 2D-peak at 2680 cm-1. With PBA present, more peaks emerge around 1140, 1240, 1400, 1500, 1560, and 1620 cm-1. These peaks have been reported previously35 as part of the Raman spectrum of pyrene. The surface concentration of PBA can then be quantified by analyzing the peak areal intensity. The four peaks in the range 14501675 cm-1 were fitted so that each peak could be studied separately, as shown in Figure 1b. The G-peak is the most important one here as it is proportional to the amount of sp2-sp2 hybridized carbon36 in a sample. By examining the ratio Rsp2 of sp2-sp2 carbon in PBA to sp2-sp2 carbon in graphene, the surface concentration of PBA can be obtained as: &'() =
*+,*./0
=
10(./02+,- ) 10(./0) 10(./0)
(1)
where NPBA and NSLG are the amounts of sp2-sp2 carbon in PBA and in SLG, respectively, while AG(SLG) and AG(SLG+PBA) represent the G-peak areas originating from the different materials indicated by the subscripts. The sp2-sp2 carbon in PBA is found exclusively in the rings which have the same packing density as graphene. Hence, since the carbon surface density in graphene is known, the PBA surface concentration can be determined. Finally, by applying the described analysis procedure to the samples with varying PBA concentrations, the adsorption isotherm of PBA on graphene is obtained. If the alkane chain causes a steric hindrance and leads to the formation of a monolayer, the Langmuir equation will be applicable to the experimental data. However, if multilayers of PBA molecules would grow, the isotherm would assume a different form, and the PBA amount on the surface would not saturate. The sp2-sp2 carbon ratio for varying PBA concentrations is fitted by the Langmuir equation below with the saturated sp2-sp2 carbon ratio Rsat as a second parameter: 67
&'() = &'34 5 = &'34 8967 (2) ACS Paragon Plus Environment
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where, 5 is the surface coverage, K is the equilibrium constant in mM-1, and C is the concentration in mM. The experimental data along with the fitting result are shown in Figure 2a. The results from the fit are &'34 = 2.2 and : = 4.5 × 10 mM 8. Graphene has a surface density of 38.2 atoms per nm2 and there are 16 sp2-sp2 hybridized C-atoms in a PBA molecule. This means that the PBA surface density is 38.2 × 2.2> = 5.3 PBA molecules per nm2 under saturation. 16
Figure 2. a) Ratio of sp2-sp2 carbon in PBA to sp2-sp2 carbon in graphene for different PBA concentrations fitted by equation (2), b) D’ peak fitted with K fixed to the values achieved in a), c) simulated energetically favorable morphology with PBA flat on graphene, and d) simulated energetically favorable morphology with 3 PBA binding to each other. If the pyrene part of PBA was lying down on graphene under saturation, Rsat would be below or at unity. The result &'34 = 2.2 indicates an orientation of the PBA molecules other than the face-to-face ACS Paragon Plus Environment
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one on graphene. As mentioned in the introduction, several probable configurations in the interaction among small aromatic molecules exist, and these can be considered in the PBA-graphene system as well. If a face-to-face binding between a PBA molecule and graphene takes place, it is most likely to be AB-stacked. An AB stacking would, however, lead to a split of the 2D-peak37,38 which is not observed in Figure 1c. What is observed instead is that as the PBA concentration increases, the 2D peak broadens with the peak area staying the same and the Lorentz shape of the 2D peak remaining unchanged. When the surface concentration of PBA is low there is space for the molecules to move around on graphene with which they mainly interact. This could lead to a surface morphology with horizontally oriented PBA molecules on graphene. As the surface concentration of PBA on graphene increases, the molecules will experience strong interactions with both graphene and other molecules that surround them. This would likely lead to a morphology where PBA binds at a relatively steep angle to graphene with an offset face-to-face interaction between the molecules. A carbon ratio of one would be achieved if the PBA molecules were lying down with their pyrene part binding to graphene and the chain part pointing away from the surface. In this case, the periodicity would be equal to six times the lattice parameter of a benzene ring (0.142 nm), i.e. 6×0.142 nm = 0.852 nm. This is the sum of the length of pyrene that is five times the lattice parameter and one extra lattice parameter to account for the distance between two adjacent molecules. If the distance between two standing PBA molecules is assumed to be equal to the graphite interlayer spacing, i.e. 0.335 nm, the maximum carbon ratio that could be achieved is &'34 = 0.852 ?>0.335 ? = 2.54. The measured carbon ratio of 2.2 may depart from this theoretical value, since the inter-PBA-molecule distance can be different from the graphite interlayer spacing and since the PBA to SLG angle may be less than 90°. The D’ peak was not measurable in our SLG, indicating that the SLG has few defects. It did, however, appear in the PBA spectrum and the D’ peak area should, therefore, be proportional to the amount of PBA. To test this, the areal dependence on concentration for the D’ peak was evaluated using the Langmuir equation (shown in Figure 2b) while keeping K constant from the fit to the G peak: The fitting 9 ACS Paragon Plus Environment
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process was carried out by varying the saturation value. For low PBA concentrations, the D/G ratio stayed at the graphene level of 0.2. For high concentrations, it saturated at 0.5 which is likely the level of PBA. However, uncertainties persist and an accurate measurement of PBA without graphene was difficult since PBA shows a strong fluorescence. The fluorescence of PBA is quenched by its interaction with graphene.39 In order to achieve an atomistic insight into the competing configurations, we performed DFT electronic structure calculations to compare the energetics of a single PBA molecule placed flat on SLG (Figure 2c) with the case in which three PBA molecules are stacked face-to-face standing vertically on SLG (Figure 2d). The results of these calculations show that the interaction energy per PBA molecule is 3.2 eV when PBA is lying flat on SLG, while it is 4.1 eV when it is in the upright stacked configuration. This finding indicates that it would be energetically preferable for PBA molecules to “stand up” from the SLG when the concentration is sufficiently high.
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Figure 3. a) Effective thickness of adsorbed PBA molecules determined by SE. The curve represents the fit with equation (2) by keeping K fixed to the values achieved in Figure 1a, and b) contact angle of a water drop for samples treated with different concentrations of PBA. The effective thickness of adsorbed PBA molecules determined by means of SE is shown in Figure 3a. With the increase of PBA concentration, this thickness increases. This behavior can be well described by the Langmuir isotherm with the same equilibrium constant obtained for the Raman data, i.e., K=4.5×10-2 .The only fitting parameter of the isotherm is the saturation thickness, which is around 0.7 nm. As depicted in Figure 1a, the pyrene and alkane chain parts of the molecule are around 0.71 and 0.5 nm in length, respectively. This implies that the total height of a fully vertically packed PBA monolayer could be in the range of 0.7 to 1.2 nm. The experimental result of saturation thickness of around 0.7 nm indicates that the PBA molecules bind at a high angle close to 90⁰ with respect to the graphene surface. This result corroborates a monolayer formation implied from the Raman analysis above and supported by the DFT calculations. The graphene surface without PBA is largely hydrophobic, mainly due to the presence of hydrophobic adsorbents.40 If the model above for the adsorption of PBA molecules prevails, the sample surface should become less hydrophobic and the contact angle of a water drop should decrease when more PBA molecules bind to the graphene surface. This is indeed observed in Figure 3b. A standing PBA molecule on graphene should be hydrophilic since its carboxylic group is expected to face out from the surface and be in contact with the water drop. The observed decrease of contact angle with increasing PBA concentration in Figure 3b indicates that the carboxylic groups of PBA are indeed facing away from the substrate.
CONCLUSIONS We have developed a new method for analysis of molecular adsorption on graphene based on Raman scattering. This method has, together with spectroscopic ellipsometry and ab initio calculations, enabled 11 ACS Paragon Plus Environment
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the conclusion that the PBA adsorption leads to the formation of a self-limiting self-assembled monolayer on graphene, with a density of 5.3 molecules/nm2 and a thickness of 0.7 nm. When the surface PBA concentration increases, the pyrene is forced to orient vertically in order to achieve the highest packing density with the lowest energy. This is highly important for functionalizing graphene by means of PBA.
AUTHOR INFORMATION Corresponding Author *Zhi-Bin Zhang,
[email protected] ACKNOWLEDGMENT The study is supported by the Knut and Alice Wallenberg Foundation (2011.0113 and 2011.0082), by the Swedish Foundation for Strategic Research (Dnr SE13-0061) and by the Swedish Research Council (VR, No.621-2014-5591). Atomistic modeling was performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Centre in Sweden (NSC) in Linköping. REFERENCES (1)
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