Detection and Manipulation of Charge States for Double-Decker

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Detection and Manipulation of Charge States for Double-Decker DyPc Molecules on Ultrathin CuO Films 2

Yajie Zhang, Yongfeng Wang, Peilin Liao, Kang Wang, Zhichao Huang, Jing Liu, Qiwei Chen, Jianzhuang Jiang, and Kai Wu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00751 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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Detection and Manipulation of Charge States for DoubleDecker DyPc2 Molecules on Ultrathin CuO Films Yajie Zhang1, Yongfeng Wang2*, Peilin Liao3, Kang Wang4, Zhichao Huang1, Jing Liu1, Qiwei Chen1, Jianzhuang Jiang4, Kai Wu1*

1

BNLMS, College of Chemistry and Molecular Engineering, Peking University, Beijing

100871, China 2

Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics,

Peking University, Beijing 100871, China 3

School of Materials Engineering, Purdue University, West Lafayette, IN 47907, USA

4

Department of Chemistry, Beijing University of Science and Technology, Beijing 100083,

China

ABSTRACT Charge states of double-decker phthalocyanine lanthanide complexes significantly influence their geometrical structures and magnetic properties. In this study, the charge states of single DyPc2 molecules on an ultrathin CuO film were detected by scanning tunneling microscopy and spectroscopy in magnetic fields. Four types of adsorptions of DyPc2 molecules on CuO were experimentally observed. Without applying voltages, two of them were positively charged with the other two at the neutral state. By controlling the sample bias, two types of neutral molecules can be switched to the positively and negatively charged states,

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respectively. This manipulation was not realized for the DyPc2 cations. The way to precisely detect the molecular charge states with and without current is beneficial for the development of molecule electronics.

KEYWORDS: charge state, DyPc2, CuO, scanning tunneling microscopy, scanning tunneling spectroscopy


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In the past decades, molecular electronics, in particular the electron transport processes at the molecular level, has been extensively explored at the interfaces of multi-disciplines due to its potential applications in nanoelectronics.1-3 It aims to make use of individual or assembled nanoscale ensembles to construct functional devices and electrical circuits.4-6 Functional molecules are anticipated to overcome the difficulties in and limitations for miniaturization of electronic devices. Many challenges need to be tackled before such a novel strategy can be eventually enforced. One key fundamental issue is to understand current transports through single molecules.7 Single-molecule magnets (SMMs) exhibit magnetic hysteresis below a certain blocking temperature at the molecular scale.8-15 They are promising in ultrahigh-density data storage16,17 and quantum computing.18-20 For application in solid state devices, the SMM molecules needs to be bridged in junctions or positioned on surfaces.16-19 After being charged, their geometric structures and spin states are modified.21-23 Consequently, these would change molecular blocking temperatures and influence their performance in devices.21-23 Thereafter, it is of high value to detect charge states of SMMs in devices with and without applied voltages. Scanning tunneling microscopy (STM) and spectroscopy (STS) are powerful tools to characterize the geometric structures, electronic and magnetic properties of single molecules at surface.24-36 The detection of the charge states of the TbPc2 molecules on Au(111) have been experimentally realized by spectroscopic measurements of Kondo resonances. However, molecular properties are strongly affected by metallic surfaces due to their strong interactions. Therefore, thin insulator films37 are needed as the supported substrates to decouple the LnPc2 molecules and investigate their charge states.


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In this study, an ultrathin CuO film was used to electronically decouple DyPc2 molecules from the metallic Cu(110) surface. Four types of adsorptions of DyPc2 molecules on CuO were experimentally observed. Due to the weak molecule-surface coupling it is impossible to ascertain molecular charge states through the spectroscopic measurement of Kondo resonances. Magnetic fields were applied to tunneling junctions to split the molecular spin-related energy level. By detecting Zeeman effects, we found out that two types of molecules were at the neutral states and the others were charged. Further detection of molecular occupied and unoccupied electronic energies indicated that the charged molecules were cations. By controlling the sample bias, two types of neutral molecules can be switched to the positively and negatively charged states, respectively.

RESULTS AND DISCUSSION A double-barrier tunneling junction (DBTJ) consists of vacuum and CuO-film barriers, as shown in Figure 1a. The DyPc2 molecule is composed of two Pc ligands with an azimuthal angle of 45o, and a sandwiched Dy atom (Figures 1b, c). The Dy atom coordinates to eight N atoms.

Figure 1. (a) Schematic of a double-barrier junction, formed by the STM tip, vacuum barrier, DyPc2 molecule, CuO-film barrier and Cu(110) substrate. (b) Top and (c) side views of a DyPc2 molecule.


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Figure 2. (a) STM topographic image of DyPc2 molecular island adsorbed on a CuO film (V = -0.7 V, I = 12 pA). Between dashed black lines are the molecular rows with three types of molecules, as shown in the dashed white square box. (b) dI/dV spectra measured with the STM tip positioned over the centers of four types of molecules in (c) and (e), in comparison to the spectrum of the oxide surface. These molecules are marked by A, B, C, D, respectively. Before opening the current feedback for spectroscopy, the STM was operated at VA = 0.6 V, VB = −0.5 V, VC = −0.8 V, VD = −1.5 V and VCuO = −1.5 V, respectively. The tunneling current was always 80 pA. The successive spectra are vertically shifted by 0.5 nS for clarity. (c) Magnified image of the white square box in (a). (d) Schematic molecular model for the STM image in (c), showing the same orientations of B and C, an azimuthal angle of 49 o of B relative to A. (e) STM image of an isolated DyPc2 molecule on CuO film (V = -1.0 V, I = 14 pA). It shows a typical eight-lobed feature.

Image sizes: (a) 13 × 13 nm2; (c) 3.4 × 3.4 nm2; (e) 3 ×3 nm2.

LnPc2 molecules usually formed checkerboard patterns at metal substrates.25-26,31,34 DyPc2 self-assembled into two types of alternating molecular rows on the CuO film, which were


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separated by black dash arrows (Figure 2a). Three types of molecules in the assembly layer could be identified by their heights and orientations, and they all showed a typical eight-lobed feature. The dI/dV spectra were measured over these molecules, as shown in Figure 2b. The area marked by a white dashed square in Figure 2a was magnified in Figure 2c, where three types of molecules were denoted as A, B and C, respectively. In the same row, molecules B and C depict an obvious brightness difference at their centers, which is similar to results in previous studies.31,34 No difference in the orientation of the upper Pc ligands was observed, in sharp contrast to the 15o rotation on Au(111)31 and 6o rotation on Pb(111)26 of two neighboring molecules. The other row is consisted of identical molecules A. Molecules B and C in one molecular row adopt the same orientation and have an azimuthal angle of 49o with respect to A (Figure 2d). The sparsely isolated DyPc2 molecule was also observed on the substrate, which was labeled as D (Figure 2e). The dI/dV spectra (Figure 2b) measured over the four types of DyPc2 molecules and bare CuO film reveal striking variations in both shape and peak position. Considering the nonprominent electronic states of an oxide film during the bias ramping, the peaks in spectra are characteristic of the intrinsic electronic states of the DyPc2 molecules. Each of the spectra for molecules A and B exhibits a broad feature very close to the Fermi level and a sharp peak caused by the electron transport through the same molecular orbital at opposite biases in the presence of DBTJ.38-42 The details will be elaborated later. A bipolar tunneling takes place through the HOMO for molecule A and the LUMO for molecule B, respectively. Molecules C and D share similar spectroscopic features with two orbitals that are ascribed to the HOMO at V < 0 and LUMO at V > 0. The energy gap is about 1.5 eV, slightly higher than the values (1 ~


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1.3 eV) on metal substrates.26,31 The molecular spectroscopic difference shown in Figure. 2b might orignate from the different molecule-surface interaction or intermolecular electronic coupling.43 Different to the isolated molecule D, the molecule C is surrounded by six molecules. Their similar dI/dV spectra indicate that the molecule-surface interaction plays the dominent role in the determination of molecular electronic structures.

Figure 3. (a) dI/dV spectra acquired at the lobe of four DyPc2 molecules under external magnetic field of B = 0 T and B = 8 T at 4.2 K. Before opening the current feedback for spectroscopy, the STM was operated at V = −10 mV and I = 450 pA. The spectra are offset for clarity. (b) Schematic showing the Zeeman splitting. ΔE is the Zeeman energy. (c) STS measured at the lobe of molecule A at different magnetic fields with feedback opened at V = 6 mV and I = 0.9 nA. (d) Magnetic field dependence of the Zeeman energy Δ. Black points were extracted from the spectra in (c). The linear fitting (red line) yields a g values of 2.1.


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The charge states of DyPc2 adsorbed on the CuO film were determined by measuring the molecular magnetic properties. According to previous studies,31,34,44 the DyPc2 molecule possesses two spin systems. One is the central Dy3+ ion and the other involves one unpaired π electron which is delocalized on two Pc ligands. The 4f electrons of Dy3+ could not be detected due to their weak coupling to the STM tip.32,34 If the ligand electron could be detected by STM the DyPc2 molecule is at its neutral state. Otherwise, it is charged. Further measurement of orbital position by STS would identify the molecular positive or negative charge states. The detections of the Kondo effect and spin excitation are two effective ways to determine molecular spin states.45 The insulating CuO film suppresses the electronic coupling between the DyPc2 molecule and metallic Cu(111) substrate. Therefore, the Kondo resonance was not observed for all four types of DyPc2 molecules without magnetic field (denoted as B), as the featureless STS curves displayed in Figure 3a. Because the energies of two spin states (ms = ±1/2) are degenerated no spin excitation was found in Figure 3a at 0 T. Under a magnetic field of 8 T, both the spectra of molecules A and B show a remarkable dip positioned at the Fermi level. These indicate that they are at the neutral states. The degenerated state splits into two levels (Figure 3b) by an energy difference, ΔE = gBB, where g is the Landéfactor and B is the Bohr magneton. The dips in spectra correspond to a spin excitation from ms = -1/2 to ms = -1/2.46 To be noted, the measured voltages in the range of several meV are far away from molecular HOMO and LUMO, and do not altered molecular charge states. The featureless spectra of molecules C and D at 8 T mean that they are positively or negatively charged. By reference to the HOMO position of neutral molecules A, the HOMOs for both molecules C and D in Figure 2b shift more negatively in energy. Such a change suggests that molecules C and


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D are positively charged. The loss of one electron makes the HOMO shift farther away from the Fermi level.47-48 To get a better understanding of the magnetic property of molecule A, high-resolution STS spectra were recorded at various field strengths (Figure 3c). From the spectra, ΔEs were extracted and their evolution with magnetic fields was displayed in Figure 3d. As expected, a linear increase of the splitting with the magnetic field strength is found with a Landé gfactor of ≈ 2.1.

Figure 4. (a-d) Topographic images of DyPc2 island at different sample bias values with a marked molecule A by dashed elliptic black curves. For less positive bias values a ring appears in the image of the DyPc2 molecule. (I = 32 pA). The scan size is 6 nm by 6 nm.

In the next step, we demonstrated the manipulation of molecular charge states after applying different voltages to the tunneling junction. Since the insulting CuO film may provide efficient electronic decoupling of the molecules from the metal substrate, STM topographic images might reflect molecular charge states at high voltages. STM images obtained at four representative voltages from 0.2 to 1.6 V were shown in Figure 4. At small voltages, the STM


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images were separated into bright and dark areas for molecule A (Figures 4b-d), looking quite different from molecules B and C. High-resolution STM image for the dark area of molecule A revealed an eight-lobe feature, which is quite similar to that acquired with a close tip-sample distance in constant height mode.34 When the voltage got smaller, the molecular dark part became larger. Further spectroscopic measurements indicated that the molecular dark and bright parts corresponded to the neutral and positively charged states.

Figure 5. (a) Magnified image of a DyPc2 molecule with charge ring taken at V = 0.4 V and I = 32 pA. (b) dI /dV spectra of 16 equidistant points along the dotted dark line in (a), with two peaks P and P′. The dotted vertical line corresponds to the imaging sample bias for (a). Spatial regions Ⅰ in (a) and (b) correspond to the neutral state, whereas in part II the molecule is positively charged. Before opening the current feedback for spectroscopy, the STM was operated at V = 0.65 V and I = 50 pA. (c) Plot showing the correlation of onset P′ and onset P, marked by red arrow in (b), for 60 different A molecules.


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The dI/dV spectra were sampled at 16 equidistant spots along the dashed line on the molecule in Figure 5a. Points 1, 9 and 16 are highlighted both on the image and spectra (Figure 5b). The energy position of the sharp feature at positive voltages, denoted as P′, changes prominently when the tip gradually moves across the molecule. The broad feature at negative voltages, denoted as P, exhibits a similar behavior, but in an inconspicuous way. Both features reach their maxima at the center of the ring and shift towards the Fermi level at the periphery of the ring. The onsets for P′ and P were statistically acquired from 60 different molecules and showed a linear relationship against each other (Figure 5c), in full accordance with the bipolar tunneling model.38-40, 49-54 The slope of the linear fitting is around 7.8, as determined by ɛz/d, where ɛ is the effective dielectric constant of the CuO film (~10.26 for bulk CuO films55, but the value for the ultrathin film in this experiment should be smaller56), z is the tip-sample distance, comparable to the thickness d of the CuO film. This can well explain the relatively large difference between the negative and positive bias voltages with respect to the Fermi level. The ratio of the shift for both P′ and P features theoretically conform to (ɛz/d)2, resulting in the small shift for P.38 The vertical red dashed line in Figure 5b corresponds to the sample bias at which the image of Figure 5a is obtained. It is apparent that the energy position for feature P′ locates at both sides of the red dashed line, which leads to appearance of the ring on the molecule. This means that the ring divides the molecule into two parts. Part I points to the spots where sharp feature voltage is beyond the sample bias, V, corresponding to the inner region of the ring. Part II refers to the spots outside the ring where the sharp feature voltage is less positive than V. The ring corresponds to the location where the resonant tunneling (peak P’) exactly occurs at the


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specific sample bias. For example, molecules A in Figure 4a are charged at all locations due to the relatively large bias.

Figure 6. Diagram showing the conduction through the HOMO of A under the positive sample bias (a) V < Vc and (b) V > Vc, respectively. Vc is determined by the equation Eo = αeV = αeVc tip

when electron tunneling occurs, where Eo is the HOMO of the initial molecule A. EF and ECu F are the Fermi level of STM tip and Cu substrate. (c) Illustrative scheme showing the charge states of four types of molecules at different sample biases.

The nature of the difference between Parts I and II of molecule A can be elucidated in Figure 6 to demonstrate how the molecular charge state is controlled by the sample bias. The


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DBTJ has the structure similar to two parallel capacitors51,54,57,58 and their corresponding voltage drops across the two junctions are shown by the tilted lines. Once the HOMO lines up with the Fermi level of the Cu(110) substrate, ECu F , at a positive bias V = Vc , one electron tunnels from the molecule to the substrate. As shown in Figure 6, the first tunneling step from the molecule to the substrate is determined by eV = Eo(1+εz/d), where Eo = αeV = αeVc is the HOMO of the DyPc2 molecule with respect to the Fermi level of the metal substrate, ECu F , in the unbiased junction. Subsequently, the second step takes place that one electron tunnels to the molecule from the tip. Here Vc represents the bias voltage at which electron conduction kicks off and its value is confirmed by the Eo. The value for ɛz/d is estimated to be 7.75 in this experiment, the factor α = d/ɛz+d is approximately equal to 0.125. It means that the voltage drop across the vacuum barrier is 7 times larger than that across the oxide film, leading to a relatively faster tunneling rate across the latter.38 Once the sample bias is below Vc, the HOMO stays below the ECu F and therefore electrons directly tunnel from the tip to the substrate, as shown in Figure 6a. The molecule is neutral and the uncharged topography becomes observable as the state of Part Ι (V ˂ Vc). Due to the much smaller tunneling rate across the vacuum barrier, the first tunneling step is much quicker than the second one, and thus the molecule becomes charged in the form of DyPc2+ as the bias increases to Vc or higher (V ≥ Vc, Figure 6b), corresponding to Part Ⅱ in Figure 5a. Similarly, the tunneling process of molecule A at a negative bias leads to a temporary charge state of DyPc2+ due to the tunneling from the molecule to the tip in the first step. However, the tunneling from the substrate to the molecule in the second step is faster, molecule A eventually stays in neutral state. The charge states of four types of molecules at different sample biases are summarized in


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Figure 6c. As mentioned above, molecules C and D are positively charged regardless of the sample bias. For molecule B, bipolar tunneling occurs through its LUMO. In analogy with molecule A, molecule B is negatively charged at a more negative bias with respect to Vc which is determined by eV = −ELUMO(1+ d/εz) and ELUMO = αeVc. At a much smaller negative or positive bias, molecule B keeps its neutral state. The result that molecules A and B are neutral around the Fermi energy, in coincidence with the high-resolution spectra under magnetic field for molecule A and B in Figure 3a. On the contrary, the measured spectra provide a practical methodology to identify the charge state of the molecule. Though A and B exhibit a similar tunneling process, the absence of the ring for molecule B may be ascribed to the invariance of its LUMO sampled at different locations above the molecule. In combination of the sample bias control and the spectroscopic measurement, detection and manipulation of the DyPc2 charge states were ultimately achieved.

CONCLUSIONS Four types of DyPc2 molecules were experimentally observed on ultrathin CuO films. Their charge states were determined by STM and STS in magnetic fields. Without applying voltages, two of them were positively charged with the other two at the neutral state. By applying the certain sample voltages, two types of neutral molecules can be switched to the positively and negatively

charged

states,

respectively.

This

manipulation was

not

realized

for

the DyPc2 cations because their electronic states are far away from Fermi level. The way to precisely detect the molecular charge states with and without current through the molecule is of fundamental importance for the development of molecule electronics. The influence of the charge states on magnetic properties of single molecule magnets needs further investigation.


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METHODS All experiments were performed in a Unisoku low-temperature STM with a base pressure below 2×10-10 torr and a magnetic field up to 11 T. The Cu(110) surface was cleaned by repeat cycles of Ar+-ion sputtering and annealing. The thin CuO film was prepared by exposure of the clean surface to 30 L (1 Langmuir = 1×10-6 storr) O2 at 800 K and subsequent annealing at 800 K for 10 mins. Then the DyPc2 molecules were thermally deposited onto the substrate at 490 K from a Knudsen-type evaporator. The oxidized surface was kept at room temperature during deposition. Afterwards the sample was transferred to the STM chamber and cooled down to 4.5 K. An electrochemically etched tungsten tip was used for STM measurements. The differential conductance (dI/dV) spectra were recorded through the standard lock-in technique in constant-height mode. A small sinusoidal modulation voltage with a frequency of 2 KHz was superimposed onto bias voltage. The modulation voltages are 10 mV in Figures 2 and 5 and 0.2 mV in Figure 3, respectively. All voltages referred to the sample bias with respect to the tip.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was jointly supported by National Natural Science Foundation of China (91527303,


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Detection and Manipulation of Charge States for Double-Decker

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