Two-Dimensional Covalent Organic Framework-Graphene Photodetectors: Insight into the Relationship between the Microscopic Interfacial Structure and Performance.

Graphene is an attractive material for photodetection and optoelectronic applications because it offers a broad spectral bandwidth and ultrafast response speed. However, because of the broad light absorption characteristic, graphene has a lack of selectivity to the wavelength, which limits the performance of graphene-based photodetectors. Here, we demonstrate a novel hybrid photodetector with monolayer graphene covered with an ultrathin film of surface covalent organic frameworks (COFs) with variable structures as the light-harvesting materials. Photodetectors based on surface COF-G show enhanced responsivity in comparison with unmodified graphene and graphene modified with monomers. The submolecular resolution of scanning tunneling microscopy allows us to get a direct insight into the relationship between the microscopic interfacial structure and the performance of the device. We prove that the enhancement in the device performance is directly related with the orderliness of surface COFs, which influences the interfacial charge transfer by tuning π-π stacking between surface COF and graphene.

Introduction

Graphene, a single layer of the two-dimensional (2D) carbon crystalline structure, is currently the center of attention because of its extraordinary electronic, optical, and mechanical properties.[1−9] Graphene can absorb photons from the visible to the infrared range because of its zero band gap characteristic, and this makes it a promising material for broadband photodetectors, and its extraordinary high carrier mobility facilitates achievement of ultrafast photoresponse,[10,11] but at the same time makes it lack selectivity to the wavelength[12] and leads to the poor performance of the device in terms of photoresponsivity.[8,13] The low photoresponsivity is mainly attributed to the limited light absorption in the one-atom layer and the short recombination lifetime of the photogenerated electron–hole pairs. To overcome this shortcoming, one should increase either the efficient absorption[14−16] or the lifetime of the photoexcited carriers by modifying graphene without degradation of its carrier mobility. Being a 2D material, the conductance of graphene is very sensitive to electrostatic perturbation by photogenerated carriers close to the surface,[17] thus graphene photodetectors can be sensitized by variable means such as optical microcavity,[18,19] colloidal quantum dots (QDs) such as PbS,[20,21] CdS,[22] ZnO[23] and SnO2,[24] or InSe,[25] and organic dyes.[26]

As an intrinsic organic semiconductor, covalent organic frameworks (COFs) with the π-conjugated structure can be used as the light-harvesting materials in optoelectronic devices.[27−33] The band gap and photoelectric properties of COFs can be precisely tuned by the topology and periodicity of the precursor. In contrast to the bulk COFs which are insoluble and difficult to process, surface COFs can grow in situ on arbitrary surfaces and exhibit high compatibility with traditional microfabrication techniques.[34,35] Benefiting from the planar geometry, it possesses great advantages for flexible devices.[36] However, the relatively low carrier mobility in COFs hinders the development of high-performance field-effect transistors and photodetectors.[32,33] Combining the high charge carrier mobility of graphene with the light-harvesting properties of surface COFs might create fascinating hybrid materials which enable fabrication of high-performance photodetectors with both high responsivity and speed. More importantly, the hydrophobic graphene surface can promote the preferred crystalline orientation of the surface COFs through epitaxial effects, thus facilitating the possible charge transfer between surface COFs and graphene.[37−39]

The intrinsic semiconducting property of surface COFs[40,41] can enable harvesting of light and forming a built-in electric field because of Fermi level equilibration to facilitate charge carrier separation in the surface COF–graphene hybrid, which may lead to a gain mechanism in graphene-based photoconductors and thus result in high-performance photodetectors. In the present work, we use on-surface Schiff-base coupling[42,43] to synthesize diverse ultrathin surface COF films on single-layer graphene and demonstrate a photodetector based on surface COF–graphene heterostructures. The interface structure of the hybrids can be fine-tuned by adjusting the concentration of reaction precursors and characterized by scanning tunneling microscopy (STM) with submolecular details, which represents a major advantage in comparison with those photodetectors based on small organic molecules and QD-graphene hybrids with a hardly known actual interface structure. Remarkably, the π–π interaction and van der Waals epitaxy effect between the surface COFs and graphene allow precise control of the relative alignment between these materials and favors charge transport between surface COFs and graphene. Submolecularly resolved STM studies on the interface structure provide direct evidence on the relationship between the microscopic interfacial structure and the photoresponse behaviors of these surface COF–graphene hybrid devices. This work demonstrates that the orderliness of the surface COFs plays an important role in determining the device performance.

Results and Discussion

The structure of building blocks and schematic illustration of surface COF synthesis are shown in Scheme . If 1 fully reacts with diamine 2, 3, and 4, it can form extended honeycomb networks with hexagonal pores on the graphene surface (Scheme b).

Scheme 1

(a) Molecular Structure of Monomers: Benzene-1,3,5-tricarbaldehyde (1); p-Phenylenediamine (2); 4,4″-Diamino-p-terphenyl (3); and 4-(2-(9-(2-(4-Aminophenyl)ethynyl)anthracen-10-yl)ethynyl)benzenamine (4) (b) Schematic Illustration of the Co-Condensation of Aldehyde and Amine Monomers Into Surface COFs

As shown in Figure and also demonstrated in our previous report, the on-surface synthesis of imine COFs at the interface is very sensitive to the concentration of monomers. For the three diamines shown in Scheme , keeping the molar ratio of aldehyde to amine monomers as 2:3, by tuning the concentration of the precursors’ well-ordered surface, COFs can be obtained at the liquid–solid interface at room temperature with almost entire surface coverage when the monomer concentration before mix is 0.003 mg g–1 (Figure a–c). However, with the increasing concentration of the monomers (the monomer concentration before mix is 1 mg g–1), the interfacial structure varies for the three surface COFs.

Figure 1

Representative STM images of surface COFs formed at octanoic acid/graphene on the copper foil (L/S) interface. (a,d) Surface COF, (b,e) surface COF, and (c,f) surface COF. The molar ratio of aldehyde to amine is 2:3, and the monomer concentration before mix is (a–c) 0.003 and (d–f) 0.1 mg g–1. Imaging conditions: (a) Iset = 98 pA, Vbias = 0.10; (b) Iset = 240 pA, Vbias = 0.15 V; (c) Iset = 30 pA, Vbias = 0.20 V; (d) Iset = 50 pA, Vbias = 0.50 V; (e) Iset = 90 pA, Vbias = 0.20 V; (f) Iset = 120 pA, Vbias = 0.40 V.

For surface COF, well-organized honeycomb structures were observed (Figure d); while for surface COF, although it mainly contains regular honeycomb networks, the pores of the network are filled with oligomers (Figure e). For surface COF, a high monomer concentration leads to densely packed disordered oligomers which cover the whole surface (Figure f). The different assembling behaviors of these systems are attributed to both the affinity of the diamine monomers toward the surface and the increased flexibility with the elongation of the backbone.

To fabricate surface COF–graphene (surface COF–G) photodetectors, single-layer graphene grown by chemical vapor deposition (CVD) was first transferred onto a SiO2/Si wafer using the classical poly(methyl methacrylate) (PMMA)-mediated method (Figure S1, Supporting Information).[3] Surface COF networks were synthesized on the surface of graphene through on-surface Schiff-base coupling at either the liquid–solid (L/S) or gas–solid (G/S) interface after Ti/Au (2.5/40 nm) electrode deposition. After synthesis, X-ray photoelectron spectroscopy (XPS) was conducted to verify the formation of imine bonds (Figure S2, Supporting Information). UV–vis spectrum revealed absorption peaks at 390, 385, and 470 nm for surfaces COF, COF, and COF, respectively (Figure S3, Supporting Information). Thus, we used a 400 nm laser to investigate the photoresponse of the surface COF–G hybrid photodetectors.

Figure a shows a simplified schematic depiction of the surface COF–G photodetector. As show in Figure b, the Dirac point (i.e., the charge neutrality point) of graphene shifts to more positive values after functionalization with surface COF, indicating that the surface COF acts as a p-type dopant which is consistent with the current–time (I–t) response characteristics (Figure ). The decrease in the device mobility with respect to the pristine device in the −50 to +40 V (Vg) region might be due to charge scattering caused by the interface structure after COF modification.[44] Because one advantage of surface COFs is that the structural regularity can be easily tuned, here, we focus on discussion of the structure dependence of photoresponsive behaviors of these surface COF–G photodetectors.

Figure 2

(a) Schematic illustration of the structure of the surface COF–G photodetector. (b) Characteristic I–Vg curves of pristine graphene (black) and surface COF–G (red) photodetectors.

Figure 3

(a,b) Typical photocurrent response of detectors of surface COF–G and (c,d) surface COF–G hybrids at the octanoic acid/graphene (L/S) interface. The molar ratio of aldehyde to amine is 2:3, and the monomer concentration before mix is (a,c) 0.1 and (b,d) 0.003 mg g–1.

Figure shows the typical photocurrent response of surface COF–G photodetectors of surfaces COF and COF at different monomer concentrations. In these devices, the surface COFs are synthesized in situ at the liquid–solid interface, which allows fine-tuning of the structure regularity. A pronounced current change was observed under irradiation (bias voltage Vds = 0.1 V, VG = 0 V). In contrast, the photoresponse of pure graphene or graphene modified with monomers is negligible under the same irradiation (Figures S4 and S5, Supporting Information), indicating that the enhancement of light responsivity comes from the surface COFs rather than the monomers. For surface COF–G photodetector, we observed a much significant increase in the photocurrent with a high monomer concentration (Figure a) compared to that with a lower monomer concentration (Figure b). However, this was inversed in the case of the surface COF–G photodetector (Figure c,d): the photodetector with a lower monomer concentration shows a more pronounced current increase. These results can be rationalized with the different structural evolution with change in the monomer concentration for these two surface COFs.[43] As shown above, surface COF forms highly ordered structures at both high and low monomer concentrations, and because it forms A–A stacking multilayers at a high monomer concentration[42,43] (Figure S6), the increased film thickness leads to stronger light absorption and the π–π stacking facilitates charge transfer between surface COF and graphene, which in turn leads to an increased photoresponse. However, for surface COF, with a high monomer concentration, only disordered oligomers form at the interface. Because of the low conjugation degree and possibly bad adsorption registration, these oligomers do not contribute so much to enhance the photoresponse of the device. Only with a low monomer concentration, well-defined surface COF forms at the interface and the photoresponse of the device is significantly enhanced. These results strongly suggest that the enhanced photoresponse is directly related to the formation of well-ordered surface COFs.

To put more in-depth insights into the relationship between the interfacial structure of surface COFs and the performance of the surface COF–G photodetector, we have investigated the photoresponse properties with the photodetectors prepared both at the liquid–solid interface at room temperature and with moderate heating under low vacuum. As shown in Figure and discussed above, the photoresponse properties of the photodetectors with surface COF/G and surface COF/G show very different trends with the change of the monomer concentration, which we attribute to different trends in the change of interfacial structures in response to the monomer concentration. Interestingly, although surface COF shows compact assembly of not fully reacted oligomers at the liquid–solid interface when the concentration of monomers is high, the photoresponse is still significantly enhanced (Figures b and S7). This is rationalized by the unique interfacial structure of this surface COF with a high monomer concentration. As shown in Figures e, S6, and S8, at a high monomer concentration, surface COF forms compact assembly composed of pitches of hexagons and zig-zag oligomers in the layer in direct contact with the graphite surface, while a well-ordered surface COF with regular hexagon structure is discovered on the top of this layer (Figure S8). This allows π–π stacking both between the first layer and the substrate and the top layer and the first layer, facilitating efficient charge transfer between graphene and surface COF. Moreover, this may be the reason for the enhanced photoresponse of the surface COF/G composites.

Figure 4

Statistical analysis of responsivity of devices of surface COF–G hybrids prepared at the L/S and G/S (after heating) interface, and the monomer concentration before mix is (a) 0.003 and (b) 0.1 mg g–1; the inset in (a) shows a magnified image.

When the surface COF/G composites were prepared under low vacuum with moderate heating, the unreacted monomers, especially the monomers with a low molecular weight, will be pumped out of the chamber and the increased temperature favors the reaction to proceed more completely.[42] Thus, under such conditions the surface COFs prepared at either low or high monomer concentrations can form regular networks (Figure S9, Supporting Information), and the influence of the monomer concentration on the performance of surface COF–G hybrids is not as significant as that at the liquid–solid interface (Figures and 5a). However, atomic force microscopy (AFM) characterization still reveals a dependence of the film thickness on the monomer concentration (Figure a), although the dependence varies with the combination of monomers. Only the response of surface COF and COF increases with increasing film thickness. This may be attributed to the inefficient charge transfer in the thicker films.

Figure 5

(a) Relationship between the film thickness and the responsivity of the devices. Surface COF, COF, and COF were represented by circles, triangles, and squares, respectively. Blue and red colors represent the different concentrations of 0.1 and 0.003 mg g–1, respectively. Statistical analysis of the response time of different COF–G devices with different concentrations before and after heating, and the concentration of the monomers before mixing is (b) 0.003 and (c) 0.1 mg g–1.

For all the three surface COFs, the response speed decreases with the increasingfilm thickness (Figure b,c), which might be caused by the longer migration distance of the charges in the thicker films. The modification of optical properties of graphene might arise from the charge transfer between surface COFs and graphene. When the surface COFs absorb light, the photogenerated holes can be injected into the valence band of graphene, while photogenerated electrons remain in the conduction bands of surface COFs. Thus, the photocurrent will be enhanced both because of the increase of number of carriers in graphene and the recombination lifetime. The enlarged domains of more ordered surface COFs allow the excitons to be delocalized[45] and more easily diffuse to the interface.

Conclusions

In summary, we have directly synthesized surface COF films with variable structures on graphene and investigated the photodetecting performance of these hybrid materials. The photodetectors based on surface COF–G heterostructures show enhanced responsivity in comparison with unmodified graphene and graphene modified with monomers. In addition, the well-controlled synthesis of surface COFs at the liquid–solid interface allows us to get a direct insight into the relationship between the microscopic interfacial structure and the performance of the hybrids with the aid of submolecular resolution of scanning tunneling microscopy. We proved that the enhancement is directly related with the formation of the well-ordered surface COF interfacial structure. The intrinsic semiconducting property of surface COFs both enables harvesting of light with selected wavelengths and facilitates charge carrier separation in the surface COFs–graphene hybrids. Our work not only opens a viable way for the potential applications of surface COFs but also provides a possible strategy for understanding the microscopic interfacial structure–performance relationship of sensitized photodetectors of other 2D materials.

Experimental Section

Materials

All the starting materials, except monomer 4, were purchased either from J&K or Aldrich and used without further purification. The monomer 4 was synthesized according to the reported methods.[46] All the solvents are of ACS grade unless otherwise noted.

Device Fabrication

The silicon wafer with a 300 nm oxidization layer was used for device fabrication. The channel length between the source and drain electrodes was around 130 μm in length and 3 mm in width. The symmetric source and drain electrodes [Ti (2.5 nm)/Au (40 nm)] were deposited on the graphene film through a shadow mask by thermal evaporation, followed by annealing in an Ar atmosphere to remove additional impurities adsorbed on graphene. The monomer solution was deposited on the graphene between source and drain electrodes with a simple drop-casting method.

Synthesis of COFs on the Graphene Device

The large-area single-layer graphene was grown on Cu foil by the CVD method and then was transferred onto a SiO2/Si wafer using the classical PMMA-mediated method.[3] The graphene was transferred onto the SiO2/Si substrate for further evaluation and applications (Figure S1a,b). The uniformity of the graphene film can be evaluated via color contrast under an optical microscope. The brighter region (white arrow) refers to the SiO2/Si substrate, and the darker color (black arrow) corresponds to the bilayer or multilayer graphene. The quality of graphene on a SiO2/Si substrate was further verified by Raman spectroscopy (Figure S1c). The two prominent peaks are G and 2D peaks, and the intensity ratio of 2D/G approximately shows typical features of the monolayer graphene.[47]

For the on-surface synthesis of COF at the liquid/solid interface at room temperature, the monomers (1, 2) were dissolved in the octanoic acid solvent with a mass concentration of 0.1 and 0.003 mg g–1, respectively. Then, 1 and 2 were mixed with a mole ratio of about (1/2 = 2:3). The monomers (3, 4) were first dissolved in dimethylsulfoxide (DMSO) with a mass concentration of 1 mg g–1 and diluted with octanoic acid to 0.1 and 0.003 mg g–1, respectively, then mixed with a mole ratio of about (1:3/4 = 2:3). After that, the mix solution was drop-casted on the graphene/SiO2/Si surface with the evaporated Ti/Au electrode. The amount of the drop-cast just covered the surface of graphene. Synthesis of COF at the gas/solid interface: the abovementioned samples were positioned in a preheated vacuum oven and annealed at 140 °C for 30 min with a base pressure of <133 Pa.

Characterization

All the measurements were carried out at room temperature and ambient conditions. The graphene photodetector was characterized with a semiconductor parameter analyzer (Keithley 2400 SourceMeter) and the I–V characteristics of the graphene photodetector were measured with a semiconductor parameter analyzer (Keithley 4200-SCS) in a dark box to eliminate the interference of ambient light.

Monochromatic light measurements were recorded using a CT 150 xenon arc lamp source monochromated by using a DK 240 monochromator. The light power intensity was measured by using an AvaSpec-ULS2048 optical power meter (Physcience Opto-Electronics., Ltd., Beijing) in the same experimental conditions. The laser spot is a square with the size 3 mm × 3 mm, while the light on-off experiments were carried out by repeated on–off switching.

STM measurements were performed by using an Agilent 5100 scanning probe microscope with mechanically formed Pt/Ir (80/20) tips under ambient conditions. All the images were taken with the constant current mode.

The AFM characterization of the COF–G hybrid was performed with a Bruker MultiMode-8, operated at room temperature and ambient conditions.

XPS was performed with an ESCALAB 250Xi electron spectrometer with a monochromatized Al Kα X-ray source (1486.7 eV).

The optical image was acquired with an intelligent semiautomatic digital microscope DM4000M. The Raman spectra were excited at 532 nm with an incident power <1 mW (laser spot diameter was 500 nm) with a Renishaw inVia plus.

The XPS and UV–vis spectroscopy experiments were prepared by the same synthetic methods as mentioned above on the graphene surface. The concentration of the monomers was 0.1 mg g–1 of all the samples for XPS and UV–vis spectroscopy experiments.