Electronic Structures of Nucleosides as Promising ... - ACS Publications

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Electronic Structures of Nucleosides as Promising Functional Materials for Electronic Devices Yungsik Youn,† Kwanwook Jung,† Younjoo Lee,† Soohyung Park,† Hyunbok Lee,*,‡ and Yeonjin Yi*,† †

Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea Department of Physics, Kangwon National University, 1 Gangwondaehak-gil, Chuncheon-si, Gangwon-do 24341, Republic of Korea

‡

S Supporting Information *

ABSTRACT: The energy level alignments of nucleosides fabricated between conventional Al and indium tin oxide (ITO) electrodes by means of a vacuum electrospray deposition technique were investigated using in situ ultraviolet and X-ray photoelectron spectroscopy measurements. The electronic structures of four nucleosidesdeoxyguanosine, deoxyadenosine, deoxycytidine, and deoxythymidinewere determined, and their interactions with Al and ITO were analyzed. When in contact with ITO, each nucleoside showed an interface dipole that reduced the work function. On the other hand, when Al was deposited on the nucleoside layers, strong chemical interactions were observed due to electron transfer from Al to the nucleosides. Compared to their nucleobase counterparts, nucleosides commonly had lower ionization energies (IEs) and electron affinities (EAs). The origin of this difference in electronic structure was analyzed with density functional theory calculations. The sugar moieties in the nucleosides were found to induce electron-donating effects on the base moiety and led to reductions in IE and EA.

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INTRODUCTION Materials originating from living things have been spotlighted as a new class of functional materials for electronic devices in the past decade because they are environmentally friendly, renewable, and inexpensive resources that have unique optoelectronic properties.1−3 For example, DNA-cetyltrimethylammonium (DNA-CTMA), a surfactant-modified DNA for thin-film coatings, has been successfully employed as a functional layer in organic light-emitting diodes (OLEDs),4,5 organic field-effect transistors (OFETs),6,7 quantum dot LEDs,8 and organolead halide perovskite solar cells.9 Nucleobases, the building components of DNA, have also been used to form buffer layers in OFETs.10 Such materials are known to play an important role in transporting and blocking the carriers in devices, thereby improving device efficiency. However, despite remarkable recent device applications of these DNA-based materials, their electronic structures have not been investigated in detail. Understanding of energy level alignments in devices is of great importance to allow the design of efficient device architectures.11,12 The lack of such an understanding presently limits the utilization of DNA-based materials in electronic devices. In a previous study, we reported on the electronic structures of nucleobases inserted between typical Al and ITO electrodes, as determined by means of in situ ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) measurements.13 UPS and XPS are powerful analysis tools to probe the occupied density of states in solid-state thin films including semiconductors and biomaterials.14−16 Nucleobases have high ionization energies (IEs) and low electron affinities (EAs), © 2017 American Chemical Society

enabling their use in devices as efficient charge blocking layers. Based on this understanding, a guanine nucleobase has been successfully adopted as an efficient hydrogen getter and chargetrapping layer in InGaZnO FETs and OFETs.17 Expanding from nucleobases, nucleosides, which are nucleobases linked to 5-carbon sugars, could also be promising candidates for functional materials in electronic devices. However, to the best of our knowledge, no electronic devices using nucleosides have been reported, unlike the case for nucleobases and DNA-CTMA. This can be attributed to the paucity of fundamental information on the electronic structure of nucleosides. As mentioned above, understanding of the electronic structures of nucleosides and determining their energy level alignment with conventional electrodes are prerequisites to the use of these materials in device applications. Furthermore, the sugar moiety in nucleosides influences the electronic structure of the molecule, so comparisons between nucleosides and their corresponding nucleobases are also necessary. A greater understanding in these areas would make available additional DNA-based building blocks having tuned electronic properties for electronic devices. Toward such an understanding, in the present work, we explored the interfacial electronic structures of nucleosides situated between conventional electrodes with in situ UPS and XPS measurements and theoretical calculations using density functional theory (DFT). We studied four nucleosides: Received: February 22, 2017 Revised: April 30, 2017 Published: May 25, 2017 12750

DOI: 10.1021/acs.jpcc.7b01746 J. Phys. Chem. C 2017, 121, 12750−12756

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The Journal of Physical Chemistry C

Figure 1. Molecular structures of the nucleosides investigated in this study.

Figure 2. UPS spectra of (a) the SEC region and (b) the HOMO region of Al (0.02, 0.05, 0.1, 0.5, 1.0, and 4.0 nm)/dG (0.5, 1.0, 1.5, and 2.0 nm)/ ITO. (c) N 1s XPS spectra of dG (2.0 nm) on ITO and Al (0.05 nm) on a 2.0 nm thick dG layer.

VESD procedure used to deposit nucleosides is given in our previous report.18 The nucleoside materials 2′-deoxyriboseguanosine monohydrate (>98% purity) for dG, 2′-deoxyriboseadenosine monohydrate (>98% purity) for dA, 2′-deoxyribosecytidine monohydrochloride (>98% purity) for dC, and 2′deoxyribose-thymidine (>98% purity) for dT were purchased from Jena Bioscience GmbH. These were dissolved in DI water/methanol mixtures (1:2 volume ratio) at 2 mg mL−1 and used for VESD. The nucleosides were deposited on ITO in a stepwise manner with VESD. The deposited nucleoside layers did not contain detectable hydrate or hydrochloride (Figure S1, see SI). During VESD, the capillary bias of 1.5 kV and injection rate of 1 mL h−1 were used, and the pressure of the preparation chamber was maintained below 2.0 × 10−5 Torr. At each deposition step, the sample was transferred to the analysis chamber from the preparation chamber without breaking vacuum, and the electronic structure was measured with UPS and XPS. The analysis chamber consisted of a PHI 5700 spectrometer and ultraviolet (He I, 21.22 eV) and X-ray (Al Kα, 1486.6 eV) light sources. Nucleoside thickness was estimated based on the attenuation length of XPS In 3d spectral features of the ITO substrate.19,20 After completing nucleoside deposition, Al was thermally evaporated onto the nucleoside layer at the slow deposition rate of 0.001 nm s−1, and the electronic structure was measured using the same analysis sequence. The deposition rate and thickness of Al were

deoxyguanosine (dG), deoxyadenosine (dA), deoxycytidine (dC), and deoxythimidine (dT) (Figure 1). To determine the applicability of each of these nucleosides in electronic devices, indium tin oxide (ITO) transparent conducting oxide was chosen as the substrate, and Al was deposited over each nucleoside after its deposition on ITO. The energy level alignment at each nucleoside/electrode interface was then established. For incremental deposition of the nucleoside layer, we employed a vacuum electrospray deposition (VESD) technique18 as an alternative to thermal evaporation, which could lead to thermal damage of the nucleosides. The IEs and EAs of nucleobases and their corresponding nucleobases were compared to determine the impacts of the sugar moieties on the electronic structure of the nucleosides.

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EXPERIMENTAL DETAILS The energy level alignments of Al/nucleosides/ITO were investigated with in situ UPS and XPS measurements. The in situ analysis system was composed of preparation and analysis chambers. The ITO substrates were ultrasonically cleaned by deionized (DI) water, detergent, acetone, methanol, and DI water again in sequence. After this wet cleaning, the ITO substrates were treated with UV-ozone and then loaded into the preparation chamber. The preparation chamber was equipped with a VESD system and was maintained at the base pressure of 5.0 × 10−8 Torr. A detailed description of the 12751


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Figure 3. UPS spectra of (a) the SEC region and (b) the HOMO region of Al (0.02, 0.05, 0.1, 0.5, 1.0, and 4.0 nm)/dA (0.5, 1.0, 1.5, and 2.0 nm)/ ITO. (c) N 1s XPS spectra of dA (2.0 nm) on ITO and Al (0.05 nm) on a 2.0 nm thick dA layer.

on the dG layer. Under low Al coverage (0.02 and 0.05 nm), the SEC decreased another 0.40 eV in kinetic energy. As the Al deposition continued, the SEC shifted 0.65 eV higher in kinetic energy. The SEC shift was saturated at 4.0 nm Al deposition, showing the work function of 4.20 eV, equal to that of bulk Al. The valence band of ITO ranged from 3 to 9 eV of binding energy (Figure 2b, bottom). As the dG layer was deposited on ITO, the unique features of dG emerged at 2.30, 5.20, and 9.60 eV, as indicated by vertical dashed lines and arrows in Figure 2b. The HOMO onset was observed at 2.30 eV below the Fermi level (EF) at 2.0 nm thickness. To estimate the band bending (Vb), the energetic shifts of the peaks at 5.20 and 9.60 eV were observed thoroughly during dG deposition. However, no Vb occurred at the interface between dG and ITO. On the other hand, when 0.05 nm Al was deposited on the dG layer, the strongest peak at 5.20 eV shifted to a 0.40 eV higher binding energy. The SEC shifted by the same magnitude, indicating that Vb of 0.40 eV occurred at the interface between Al and dG. This is attributed to the n-doping effect arising from electron transfer from Al to dG. With further Al deposition, the dG features were gradually broadened and attenuated, and finally the step-like EF structure of Al was seen at 4.0 nm Al thickness. To investigate the core level shift and chemical interaction between Al and dG, we performed XPS measurements. Figure 2c shows N 1s core level spectra of dG (2.0 nm) on ITO and of Al (0.05 nm) on a 2.0 nm thick dG layer and their deconvolution into three components using a Voigt function consisting of appropriate Gaussian and Lorentzian widths (fitting procedure and parameters are presented in SI): one imino N1 at 400.0 eV (−N, blue-filled) and two amino N2 at 400.7 eV (−NH2, green-filled) and N3 at 401.5 eV (−NH− or −NC−, red-filled). The intensity ratio of N1:N2:N3 was 2:1:2, matching well its stoichiometry. With 0.05 nm Al deposition, a new component N4 (gray-filled) emerged at 399.8 eV, indicating the Al−dG complex formation due to electron transfer from Al to dG.13,25 In the literature investigating the

monitored using a quartz crystal microbalance. To obtain the secondary electron cutoff (SEC) in UPS spectra, a sample bias of −10 V was applied. The spectral broadenings of the current measurements, which are attributed to thermal broadening and the contributions of the spectrometer and light source, were estimated as 0.09 eV (half width of Fermi step) for UPS and 0.9 eV (fwhm of Au 4f7/2) for XPS. To investigate the molecular electronic properties of the nucleosides, density functional theory (DFT) calculations on single molecules were performed using the Becke’s 3-parameter exchange and Lee−Yang−Parr correlation (B3LYP) hybrid functional and the 6-31G(d) basis set implemented in the Gaussian 09 package.21−23 The molecular geometries were fully relaxed, and the energetic minima were confirmed by means of vibrational frequency analysis. The simulated density of states was generated by convoluting Gaussian functions of 0.5 eV broadening applied at the calculated Kohn−Sham energy levels. Since organic molecules have weak van der Waals interactions at the film, the molecular electronic structure is a good representative of the solid-state electronic structure.24 The measured UPS spectra of nucleosides were compared to the simulated densities of states with a proper rigid energetic shift and were found to be in excellent agreement (Figure S2, see SI).

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RESULTS AND DISCUSSION Figures 2a and b show the UPS spectra of the SEC region and the highest occupied molecular orbital (HOMO) region of Al (0.02, 0.05, 0.1, 0.5, 1.0, and 4.0 nm)/dG (0.5, 1.0, 1.5, and 2.0 nm)/ITO. To accurately determine the SEC and HOMO onset, the SEC region spectra were normalized, and the Shirleytype background and He Iβ features were removed from the HOMO region spectra. The work function of ITO was measured to be 4.45 eV (Figure 2a). As a dG layer of 2.0 nm was deposited on the ITO, the SEC gradually shifted toward lower kinetic energy by 0.50 eV, indicating that the work function was decreased. After dG deposition, Al was deposited 12752


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Figure 4. Energy level diagrams of (a) Al/dG/ITO, (b) Al/dA/ITO, (c) Al/dC/ITO, and (d) Al/dT/ITO. ΨITO and ΨAl are the work functions of ITO and Al, respectively; ΔΨ is the work function change; IE is the ionization energy; Vb is the band bending; and eD is the interface dipole (unit: eV).

nucleoside/nucleobase adsorption on well-defined single-crystal surfaces, the reaction site can be resolved from the changes in measured spectra.26,27 However, in the current case of Al deposition on nucleoside films, the extremely high reactivity of Al should induce a strong and random reaction whole through the nucleoside. This gives significant spectral broadening and makes it hard to specify the reaction site. Thus, we maintained the intensity ratio of N peaks with Al deposition reflecting random reaction site and added a N4 component associated with the Al−nucleoside complex. As a result, the original dG components were shifted toward higher binding energies by 0.3 eV, a similar shift to that observed in the UPS spectra. The slight difference in the energetic shifts can be attributed to their different spectral broadenings and probing depth. Figures 3a and b show UPS spectra of the SEC region and the HOMO region of Al (0.02, 0.05, 0.1, 0.5, 1.0, and 4.0 nm)/ dA (0.5, 1.0, 1.5, and 2.0 nm)/ITO. As the dA layer was deposited on ITO, the SEC gradually shifted from the 4.45 eV cutoff of ITO toward lower kinetic energy, by 0.75 eV for the 2.0 nm thickness. As Al was deposited on the dA layer, the SEC shifted 0.40 eV further toward lower kinetic energy at low coverage (0.02 and 0.05 nm). Then, the SEC shifted 0.90 eV toward higher kinetic energy with 4.0 nm Al deposition, to the bulk work function of Al. This SEC shift tendency is similar to that observed in the Al/dG case, as displayed in Figure 2. In Figure 3b, the characteristic valence features of dA are shown at 2.80, 5.50, 7.50, and 9.70 eV with vertical dashed lines and arrows. The HOMO onset was observed at 2.80 eV below EF

for the 2.0 nm thick dA layer. It was not possible to estimate the spectral shift reliably from the emission feature at 5.50 eV due to the strong spectral overlap with the O 2p band of ITO at the binding energy of ∼5 eV. The valence features at 7.50 and 9.70 eV did not show any shifts in binding energies during dA deposition, indicating no Vb in the dA layer on ITO. As Al was deposited on the dA layer, the characteristic features of dA shifted to a 0.20 eV higher binding energy, the same direction as the SEC shift but smaller. We analyzed the chemical interaction between Al and dA with XPS measurements in the same manner as in the dG case. Figure 3c shows N 1s core level spectra of dA (2.0 nm) on ITO and of Al (0.05 nm) on a 2.0 nm thick dA layer and their deconvolution into three components: one imino N1 at 400.1 eV (−N, blue-filled) and two amino N2 at 400.9 eV (−NH2, green-filled) and N3 at 401.9 eV (−NC−, red-filled). The intensity ratio of N1:N2:N3 was 3:1:1 corresponding with its stoichiometry. With 0.05 nm Al deposition, the new Al−dA complex component N4 was observed at 399.8 eV, similar to that for Al−dG. The original dA features were shifted toward higher binding energies by 0.2 eV, the same shift observed in the UPS spectra. We also conducted the same measurements on Al/dC/ITO and Al/dT/ITO interfaces as shown in Figures S3 and S4 (see SI). Both dC and dT also showed similar energy level shifts with the dG and dA cases: The SEC shifts toward lower kinetic energy with dC and dT deposition on ITO, and the SEC and HOMO shift toward lower binding energy at the initial Al 12753


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The Journal of Physical Chemistry C deposition stages, and the SEC shifts toward higher binding energy with further Al deposition. The only difference is the amount of shifts which is somewhat larger as compared with dG and dA. This indicates that the electronic interactions of all nucleosides with Al and ITO are essentially the same. In addition, in the Al 2p core level spectra, the interface states due to electron transfer are commonly observed in all cases (Figure S5). This also corroborates the Al−nucleoside complex formation. The energy level alignments of Al/nucleosides/ITO as derived from the spectral changes in UPS measurements are shown in Figure 4. The lowest unoccupied molecular orbital (LUMO) levels of nucleosides above EF were estimated by subtracting the measured HOMO level from the optical energy gap (Figure S6, see SI). In general, dC and dT have deeperlying energy levels compared to dG and dA. At the interfaces between each nucleoside and ITO, no Vb was observed due to the negligible charge transfer between them. Instead, an interface dipole (eD) was formed at thermal equilibrium between the charge neutrality level (CNL) of each nucleoside and the EF of ITO.28−30 The eDs on ITO were measured to be 0.50 eV for dG, 0.75 eV for dA, 1.05 eV for dC, and 0.85 eV for dT for the nucleoside layer thickness of 2.0 nm. According to the magnitudes of the eDs, the CNLs of the nucleosides followed the order of dG > dA > dT > dC. The tendency of the CNL between purine-based nucleosides (dG and dA) and pyrimidine-based nucleosides (dC and dT) is similar to that of the nucleobases.13 The reduced effective work function of ITO was thus 3.95 eV for dG, 3.70 eV for dA, 3.40 eV for dC, and 3.60 eV for dT. This implies that the nucleosides could be used as interlayers to lower the work functions of bottom electrodes for inverted OPVs, playing a similar role as electrolytic or zwitterionic functional layers.31−33 However, the electron injection barriers between ITO and the nucleosides are quite large: 1.75 eV for dG, 1.45 eV for dA, 0.85 eV for dC, and 0.90 eV for dT. Therefore, the nucleoside layers would need to be ultrathin to allow their application as electrode modifiers.34 In contrast to the nucleoside/ITO interfaces, the interfaces between Al and nucleosides showed strong chemical interactions, including downward Vb. The Al atom chemisorbs to nucleosides, which leads to the formation of Al−nucleoside complexes.13,25 As a result, the electron injection barriers between the Al and the nucleosides are decreased. However, the lowest barrier in the case of dC was 0.50 eV, which should be decreased further for efficient electron injection. It is important to understand the impacts of the sugar moiety on the electronic structure of each nucleoside through comparison with its corresponding nucleobases. Figure 5a shows the IEs and EAs of nucleobases [guanine (G), adenine (A), cytosine (C), and thymine (T)] and their nucleoside counterparts (dG, dA, dC, and dT), calculated from UPS and UV−vis absorption measurements (Figures S7 and S8, see SI). Compared to the nucleobases, the nucleosides have similar electronic structures, high IE, and low EA. However, with the sugar link, definite decreases in IE and EA were observed in all cases: for example, the IE of T is 7.10 eV, whereas that of dT is 6.80 eV; the EA of T is 2.85 eV, whereas that of dT is 2.70 eV. This can be clearly attributed to the influence of the sugar moiety on the electronic structure. To explain this result, we carried out DFT theoretical calculations on nucleoside molecules. Figure 5b shows the HOMO and LUMO configurations of the four nucleosides. The HOMO and LUMO of all nucleosides were primarily

Figure 5. (a) IEs and EAs of nucleobases (G, A, C, and T) and nucleosides (dG, dA, dC, and dT) from UPS and UV−vis absorption measurements (unit: eV). (b) HOMO and LUMO configurations of nucleosides from DFT calculations.

distributed over the base moieties, with some contributions from the sugar moieties. To quantitatively evaluate the effects of the sugar moieties, we conducted molecular orbital analysis; Table 1 summarizes the results. For the HOMOs of dG and dA, the sugar moieties contributed only 0.4%; that is to say, the sugars did not meaningfully affect the IEs compared to those of the G and A counterparts. However, for all other cases, various contributions from the sugar moiety within the range of 3−8% were calculated. These contributions reduce the IE and EA (by 0.1−0.3 eV) of all nucleosides studied, except the IE of dG and dA. The charge distributions for the base and sugar moieties on a nucleoside molecule were also evaluated by means of Mulliken population analysis; Table 2 lists the results. According to this analysis, the sugar moieties commonly exhibit relatively positive electronic polarity; thus, the base moieties are of relatively negative electronic polarity within the total neutral nucleoside molecules. Thus, the energy levels of the base moiety are raised by the electron-donating sugar moiety. This subtle energetic difference would be important in fine-tuning of the energy level for favorable orbital alignments in electronic devices.

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CONCLUSION We determined the energy level alignments of Al/nucleoside (dG, dA, dC, and dT)/ITO structures with in situ UPS and XPS measurements. On ITO, eDs of negative direction were observed for all nucleosides. The reduced work functions owing to nucleoside layers ranged from 3.40 to 3.90 eV at 2.0 nm nucleoside thickness, matching well with the LUMO level of ntype organic semiconductors and leading to facile electron injection. Therefore, these nucleosides may potentially be used as efficient electrode modifiers for inverted OPVs. On the other hand, in contact with Al, all nucleosides showed strong chemical interactions. As a result, Al−nucleoside complexes were formed, and the energy levels shifted downward due to electron transfer from Al to the nucleosides. The measured IEs of the nucleosides were 6.25 eV for dG, 6.50 eV for dA, 6.70 eV for dC, and 6.80 eV for dT, and the EAs were 2.20 eV for dG, 12754


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Table 1. Contributions of Base and Sugar Moieties to the HOMO and LUMO of Nucleosides from Molecular Orbital Analysis dG LUMO (%) HOMO (%)

dA

dC

dT

base

sugar

base

sugar

base

sugar

base

sugar

96.6 99.6

3.4 0.4

97.1 99.6

2.9 0.4

93.7 93.2

6.3 6.8

94.0 92.0

6.0 8.0

Table 2. Mulliken Charges of Base and Sugar Moieties on Nucleosidesa dG charge (e) a

dA

dC

dT

base

sugar

base

sugar

base

sugar

base

sugar

−0.25

0.25

−0.26

0.26

−0.26

0.26

−0.27

0.27

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2.25 eV for dA, 2.55 eV for dC, and 2.70 eV for dT. Compared to the corresponding nucleobases, in nucleosides, the sugar moiety induces an electron-donating effect on the base moiety, which lowers the IE and EA. This study provides fundamental information on the electronic structure of nucleosides, which opens new possibilities for future applications of nucleosides in electronic devices.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01746. Wide-region XPS spectra of nucleoside films, simulated densities of states of nucleosides with DFT calculations, UPS/XPS spectra of Al/dC/ITO and Al/dT/ITO, fitting procedure and parameters used in N 1s XPS spectra, Al 2p XPS spectra, Tauc plots of nucleoside and nucleobase films from UV−vis absorption, and UPS spectra of nucleobases (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.L.). *E-mail: [email protected] (Y.Y.). ORCID

Soohyung Park: 0000-0002-6589-7045 Hyunbok Lee: 0000-0002-3046-1524 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (Grant Nos. NRF-2015R1C1A1A01055026 and 2012M3A7B4049801) and by Samsung Display Company.

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REFERENCES

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