Reduced Graphene Oxide-Nanostructured Silicon Photosensors With

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Reduced Graphene Oxide-Nanostructured Silicon Photosensors With High Photoresponsivity at Room Temperature Anagh Bhaumik, and Jagdish Narayan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00084 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Nano Materials

Reduced Graphene Oxide-Nanostructured Silicon Photosensors With High Photoresponsivity at Room Temperature

Anagh Bhaumik1, Jagdish Narayan1* 1Department

of Materials Science and Engineering, Centennial Campus

North Carolina State University, Raleigh, NC 27695-7907, USA *Correspondence to: [email protected]

KEYWORDS Nanostructures, Raman spectroscopy, Reduced graphene oxide, Variable range hopping, Pulsed laser annealing. ABSTRACT We have created nanostructured Si (~3 nm) with a direct band gap of 1.37 eV on electrically conducting reduced graphene oxide (rGO) for a highly efficient photosensor. This robust photosensor is fabricated using a non-equilibrium processing route, where nanosecond excimer laser pulses melt the alternating layers of Si and amorphous carbon to form micropillars and nanoreceptors of Si on rGO layers. The incident white light generates free carriers in the Si 1 ACS Paragon Plus Environment

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microstructures and nanoreceptors which are ballistically transported (via rGO layers) to the external circuit under the application of a voltage bias. The responsivity of rGO-Si devices to light (resistance vs time) and I-V measurements indicate an exponential drop in resistance with the incidence of white light and non-rectifying nature, respectively. Photoresponsivity of the rGO-Si devices is calculated to be 3.55 A/W at room temperature, which is significantly larger than the previously fabricated graphene-based Ohmic photosensors. Temperature-dependent resistance measurements of rGO-Si structures follow Efros-Shklovoskii variable range hopping (ES-VRH) electrical conduction in the low-temperature region (100 K). In rGO, the localization length, hopping energy, and activation energy are calculated to be 17.58 m, 3.15 meV, and 1.67 meV, respectively. The 2D nature of highly reduced and less defective rGO also render an interesting negative magnetoresistance (~2.5 %) at 5 K, thereby indicating potential implications of rGO-Si in opto-spintronics. The large-area integration of rGO-Si structures with sapphire employing nanosecond pulsed laser annealing and its exciting photosensing properties will open a new frontier for further extensive research in these functionalized 2D materials.

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Graphene is a highly desirable material for optoelectronic applications due to its high electron mobility (~200,000 cm2V-1s-1), optical transparency (~97%), electric current carrying capacity (>1018 Acm-2), fast carrier dynamics, Young’s modulus (0.5 TPa), Fermi velocity (2.5×106 m/sec), and linear energy dispersion relation.1-3 The strong interband transitions due to high electron mobility in graphene render it a potential material for ultrafast detection, FETs, memory devices, and energy storage applications.4 Graphene-based optoelectronic devices employ Schottky-junctions, which cause the formation of a built-in potential thereby separating the electron-hole pair.5 In comparison to metal silicide photosensors, graphene devices can be fabricated using a low vacuum and low-temperature processes.6 To increase the built-in potential in Si-graphene photosensors, chemical doping has also been performed.7 This introduces defects in the structure of graphene, which act as scattering centers thereby reducing the charge transfer capability of graphene. The chemical doping is also not stable and it causes the formation of an oxide layer on Si. It should also be noted that Ohmic-junctions are preferred (over Schottky junctions) due to their non-rectification nature and high-frequency operation. Since graphene is a semi-metal, it tends to form Schottky junctions with semiconductors. Therefore, reduced graphene oxide (semiconductor) can replace graphene (semi-metal) as a better optoelectronic material due to its formation of Ohmic junction (when work function rGO < work function Si) with Si structures. Large-area graphene has been synthesized using high-temperature chemical vapor deposition techniques and transfer of graphite onto various substrates via chemical route (PMMA) or mechanical exfoliation (scotch-tape method). The above-mentioned processes leave impurities and defects (on the basal plane) of graphene, thereby thwarting its use in optoelectronic devices. Therefore, novel carbon-based materials and processing techniques are needed to fabricate highly efficient and robust photosensors.



Formation of reduced graphene oxide (rGO) after chemical functionalization of graphene oxide (GO) has generated potential research interests due to high electron mobility (372 cm2V1sec-1)

and formation of optical band gap.8 The GO has low electrical conductivity due to the

presence of -COOH, -O- and -OH functional groups in its basal plane. Therefore, harsh chemical reducing agents and high temperatures are used to convert GO into rGO.9-11 The rGO films synthesized by solution-based and CVD methods contain large area grain boundaries and lack wafer-scale epitaxial integration thereby limiting the usage of rGO in optoelectronic devices. In addition, the reduction of GO into rGO creates disorder and electron localization-hopping phenomenon (Efros-Shklovskii range hopping (ES-VRH)) plays a significant role in determining its electronic properties. In rGO, the functionalizing molecules (residual oxy groups) introduce energy level in the form of edge states and states in the electronic band structure thereby rendering it to be a p-type semiconductor.12,13 Though Si is a highly desirable material for optoelectronic applications but its usage is thwarted due to the indirect nature of interband optical transitions. The direct band gap semiconductors have better optoelectronic responsivity as it involves phononless transitions. Previous reports indicate an increase in photoluminescence signal with nanostructuring Si, but at the cost of an increase in FWHM of the PL spectra.14 It should also be noted that, with nanostructuring of Si (increase in surface area), there is a tendency of formation of SiO2 layer, which creates a compressive strain.14 The focus of this research is on laser annealing technique to create novel materials with unique properties.15,16 These materials can also be integrated using a single-step strategy to fabricate high-speed multifunctional electronic devices. This research addresses a single-step integration of large-area rGO thin films with direct band gap Si structures to form a highly receptive photosensor, which is Ohmic in nature. An ideal photosensor should possess fast response-recovery time and high


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sensitivity in white light. These features are integrated into rGO-Si photosensors, which are formed after pulsed laser annealing of amorphous carbon and Si layers deposited on c-sapphire substrates. The pulsed laser annealing is an ultrafast technique (completed in less than 200 ns) and is performed at room temperature in air at ambient pressure without any reducing agent. During the PLA technique, the amorphous carbon and Si layers are melted and quenched rapidly, thereby forming a composite structure consisting of microstructures of Si and large-area rGO layers, on which 3 nm wide nanostructures of Si are formed. Temperature-dependent resistance and magnetoresistance measurements also indicate a variable range hopping characteristics and negative magnetoresistance nature (spin-orbit interactions) in rGO-Si devices. Therefore, the fabricated photosensors can also be activated using an external magnetic field. The novel photosensors synthesized using the highly non-equilibrium technique points toward a promising new direction for fabricating graphene-based non-rectifying devices. RESULTS AND DISCUSSION By pulsed laser annealing of the C/Si composite structure, we have created a unique structure consisting of micro-Si pillars on the top of rGO and formation of nanostructured (3 nm wide) Si features, which can act as direct band gap photosensors (schematically shown in figure 1). The detailed structure-property correlations of the carbon nanostructures, which are formed after the PLA, indicate similar characteristics to that observed in rGO thin films. Previous reports have also shown the synthesis of rGO and graphene-like structures from non-GO structures.17-19 Figure 2(a) depicts the room-temperature unpolarized micro Raman spectra of Si-amorphous carbon layered structure before PLA and rGO-Si structures formed after PLA. As it is evident from the figure that there occurs a significant change in the nature of Raman active vibrational modes situated between 1000-2000 cm-1. The Raman spectrum shows D, G, 2D, and D+G 5 ACS Paragon Plus Environment


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vibrational modes of rGO. The C-sp2% in the as-deposited layered structure (before PLA) is calculated to be ~60% and rest sp3. The D-peak originates due to the A1g symmetry phonon near K zone boundary. This vibrational mode involves the breathing modes of sp2 carbon atoms present in the rGO and amorphous carbon structure and occurs due to the presence of defects and asymmetry in the graphene lattice. There is a red-shift of the D peak (~13 cm-1) to 1342 cm-1 in the rGO structure (as compared to free-standing rGO flakes) formed after PLA technique. This might be due to stress generated during the PLA process and quantum interference20 by nanostructures of Si. The G peak is located at 1573 cm-1 and is due to the E2g phonon mode in the Brillouin zone center. This vibrational mode signifies bond stretching of sp2 C atoms for both rings and chains. The red-shift of G peak in rGO structure as compared to that in GO indicates removal of functional groups in rGO, which causes the stiffening of G mode. The magnitude

(11 ―+ 𝜗𝜗)Δ𝜔 𝜔

Raman shift can be correlated to the residual stress (𝜎) generated in a film using: 𝜎 = 2𝐺 , where Δ𝜔 is the shift in Raman wavenumber (7 cm-1), 𝜔0 is the Raman wavenumber corresponding to the reference state (in this case the G vibrational mode in amorphous

carbon=1580 cm-1), 𝐺 is the shear modulus and 𝜗 is the Poisson’s ratio of rGO. By considering the value of 𝐺 as 207.6 GPa, 𝜗 as 0.197, the residual stress is calculated to be 2.74 GPa (compressive in nature). This residual stress also causes blue-shift of the D peak as mentioned earlier. It should also be noted that there is no asymmetry of the G peak in rGO structure (no extra peak at 1600 cm-1), which indicates the absence of crystallite bulk graphite in the rGO-Si composites. It is well known that for GO and rGO structures the ratio between the peak intensities of D and G peak indicates the degree of reduction. According to the Tuinstra and Koenig (TK) relationship, with increasing the disorder in the structure, ID/IG increases.21 However, further reduction reaction creates more disorder and distortion in the sp2 structure. As 6 ACS Paragon Plus Environment

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a result, ID decreases compare to IG and TK relationship no longer holds true. As it is evident from figure 1(a), the rGO-Si Raman spectrum shows a higher G peak intensity as compared to D peak (ID/IG=0.883) and there is also a decrease in FWHM of the G peak as compared to chemically or thermally prepared rGO. These factors signify a better quality and a higher degree of reduction in the rGO structure formed via PLA route. The Raman spectrum of rGO-Si also shows 2D vibrational mode (at 2685 cm-1, shown as inset of figure 2(a)), the second overtone of D peak. This peak (2D) is the characteristic of double resonance transitions resulting from the generation of two phonons with opposite momentum (+k and -k) to each other. Therefore, the presence of 2D peak indicates a restoration of C-sp2 entities in the rGO structure. The intense and narrow 2D peak also indicates less number of defects (on the basal plane) in the rGO structure. The inset in figure 2(a) also depicts that 6 Lorentzian peaks can be used to fit the 2D peak which indicates 5 layers of rGO.22 The intensity ratio of 2D/G (0.75) is a measure of charge carrier mobility (in graphene, intensity of G peak is less than 2D peak therefore indicating high charge carrier mobility). Therefore, the rGO structure formed after PLA process has better electrical transport properties (ballistic transport of electrons) and is crucial for the responsivity rGO-Si photo sensing devices. Figure 2(b) depicts XPS survey scan and figures (c-e) depict high-resolution XPS scans of rGO-Si structure formed after PLA technique. The inset of figure 2(b) indicates TOF-SIMS profile of as-deposited alternating layers of C and Si (total thickness=500 nm) on sapphire (before PLA). For better laser-material coupling, amorphous carbon is engineered to be the topmost layer of the layered structure. This facilitates melting of C and Si after the incidence of nanosecond ArF laser thereby forming the rGO-Si photosensors. The survey scan indicates the presence of C 1s, O 1s, and Si (1s and 2p) peaks. The curve fittings of the high-resolution spectra



are performed using Pseudo-Voigt peak fitting function after performing a Shirley background correction. As it is evident from figure 2(c), large FWHM and broad tail of C1s spectrum indicate contribution from a variety of carbon bonding configurations. For the C 1s spectrum after deconvolution, peaks corresponding to 284.8 eV, 286.2 eV, and 288.1 eV binding energies are observed. The first peak (at low binding energy) indicates C-C bonding (sp2), whereas the other two peaks are assigned for C-O and C=O bondings.23 Since the photoelectron kinetic energies of C1s is larger than O1s, therefore, the sampling depth (for C 1s) is larger. For the case of GO, the intensity of C=O and C-O-C peaks are comparable to that of C-C XPS peak.24 So, the low intensity of C=O bonding states indicates the formation of reduced GO and restoration of delocalized π conjugation was restored in the rGO structure, which will facilitate the ballistic transport of electrons. The percentage of the sp2 C is calculated as 50.5% using 25: 𝑠𝑝2 = 𝐴𝑟𝑒𝑎 (𝐶 ― 𝐶) 𝐴𝑟𝑒𝑎 (𝐶 = 𝑂) + 𝐴𝑟𝑒𝑎 (𝐶 ― O)

× 100. The high-resolution O 1s XPS spectra (figure 2(d)) indicates the

presence of C=O (532.2 eV) and C-OH (536.7 eV) peaks. The C/O ratio of rGO structure is calculated as 4.06, which is comparable to rGO prepared using chemical and thermal processing routes. This large C/O ratio demonstrates reduction of the GO structure (formation of rGO) along with restoration of sp2 C (less defects on the basal plane). It should also be pointed out that during thermal reduction of GO (~10000C), high pressure (~130 MPa) is created in the stacked GO layers which causes a complete disruption and loss of sp2 C.26 Only 2.5 MPa is required to cause delamination of GO layers. There also occurs a 30% loss of the GO mass during the thermal reduction thereby rendering the rGO structure with vacancies (scattering centers) and topological defects (and reduced charge carrier mobility). Therefore, the current process helps in the formation of highly reduced GO with a smaller number of scattering centers, which assists in ballistic transport of electrons. The high-resolution XPS scan for Si 2p spectra (figure 2(e)) 8 ACS Paragon Plus Environment

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indicates the presence of Si-Si (100.3 eV) and Si-O (102.4 eV) bonding states in rGO-Si composite structure. The atomic% of Si-O bonding state is less than 5%, which indicates very small amount of SiO2. The absence of interfacial layers of SiO2 (between Si and rGO) causes formation of Ohmic contacts, which are required for high frequency opto-electronic operations. Figure 2(f) shows the room-temperature photoluminescence spectroscopy of Si (in the rGO-Si structure). The PL spectrum shows a sharp peak centered at 903 nm corresponding to the radiative optical transition of Si. The nature of the optical transitions (direct or indirect) can be derived by plotting (intensity of PL*hn vs h where n=1 for direct optical transition and n=2 for indirect optical transition. The linear behavior of the PL data (right inset of figure 2(f)) indicates direct band gap in the Si structure formed after PLA technique. The linear extrapolation to the x-axis depicts the direct band gap to be 1.37 eV (as compared to indirect band gap of 1.12 eV in bulk Si). It is well known that direct band gap transitions are needed for efficient emitters (fast optoelectronic response). Therefore, the formation of direct band gap (phononless transitions) in Si is essential for the fast responsivity of the fabricated rGO-Si photo sensing devices. The exploitation of Si in photonic devices is thwarted due to the indirect nature of the interband optical transitions.27 Previous research indicates the conversion of indirect to direct band gap in Si is achieved by: (a) introduction of defects in Si which causes the folding of Brillouin zone, (b) nanostructuring of Ge/Si alloy, and (c) formation of nanosized Si (~3 nm).28,29 In the present case, the blue-shift in PL spectra is due to quantum confinement (reduction of size) in the Si structures after PLA. The quantum confinement effect causes the radiative optical transitions (direct band gap) in the Si structure, which is essential for better optoelectronic activity. Due to the abovementioned effect, Γ15 shifts down in the energy level (situated at Γ). There also occurs a upshift of Δ1 (situated at X).30 The quantum confinement causes the



formation of Γ15 as the conduction band minimum (CBM) and Γ25/ as valence band maximum (VBM) in Si (in rGO-Si structures, shown as an inset in figure 2(f)). This also causes the shift of the k-space center of gravity of the electron population in the CB from Δ1 to Γ15.31 However, for very small sized Si particles ( 100 K) and at lower temperatures (< 100 K) the rate of change of resistance is fast. Therefore, at high and low-temperature regimes the electrical transport mechanism is dominated by band gap (Arrhenius conduction) and variable range hopping (ESVRH), respectively. This change in electrical conduction type indicates that the current is flowing in the well-connected rGO thin films, where variable range hopping is observed at low temperatures.12 The rGO-Si thin films follow Arrhenius conduction for T > 100 K and ES-VRH conduction for T < 100 K. At sufficiently low temperatures, activated type of conduction is not possible and therefore variable range hopping mechanism plays a significant role in the conduction process. The discontinuous sp2 clusters in sp3 matrix play a major role for the



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reduced electrical conductivity in GO structures. The electrical conductivity is improved after reduction of GO into rGO due to the creation and growth of new sp2 clusters at the expense of defects (-OH, -COOH, etc.).52 The formation of new sp2 clusters provides percolation paths between sp2 domains already present. After the PLA, new sp2 clusters are formed, which help in improving the electrical conductivity and causes variable range hopping at low temperatures. At high temperature regime (figure 7(b)) our data fits best to Arrhenius conduction model (ln(R) vs T-1.0) as compared to ES-VRH conduction model (ln(R) vs T-0.5) at low-temperature regime (figure 6(c)). ES-VRH conduction model considers Coulomb interaction between sp2 clusters 𝑇0 1/2

and is expressed using the equation: 𝑅 = 𝑅0 𝑒

(𝑇)

, where T0 denotes the characteristic

temperature and the exponent ½ is independent of dimensionality of the sample. The localization 𝛽𝑒2

length, ξ in rGO-Si thin films can be calculated using the equation: 𝜉 = 4 𝜋𝜀𝜀0𝑘𝐵𝑇0, where kB is the Boltzmann constant, e is the charge of an electron, 𝜀0 is the vacuum permittivity, and 𝜀 (=3.5) is the effective dielectric constant, and 𝛽 = 2.8 is a constant. In the rGO-Si composite thin films, the localization length is calculated to be 17.58 m. This large value of localization length denotes the formation of rGO having large sp2 entities that favor a ballistic transport of electrons, which is ideal for photo sensing applications. To facilitate fast conduction of photogenerated carriers, hopping energy (𝐸ℎ) is an important parameter, which is also calculated by ES-VRH model (for 2D materials) using the equation: 𝜎 𝑇 = 𝐴 𝑒

𝐸ℎ 𝐵𝑇

―𝑘

, where A is a constant, and 𝜎

denotes the conductivity of rGO-Si composite thin films. The hopping energy determines electron conduction by variable range hopping phenomena in-between sp2 clusters in a matrix of sp3 (scattering centers). In the present case, 𝐸ℎ is calculated to be 3.15 meV, which is extremely low (value of 𝑘𝐵𝑇 at 300 K is 25 meV) and favors an ultrafast conduction of photogenerated 18 ACS Paragon Plus Environment

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electrons. The plotted data ln(R) vs T-1 fits best to a straight line (Arrhenius conduction) in figure 7(c). Therefore, at high temperature (T > 100 K) regime band-gap dominated electronic transport occurs in rGO-Si thin films. The Arrhenius conduction is mathematically represented using the 𝐸𝑔 𝑘𝐵𝑇

equation: 𝑅 = 𝑅0 𝑒 . Using the slope of the straight line (in figure 7(c)), the activation energy is calculated to be 1.67 meV, which is comparable to the previously reported values in rGO structures.9 The low values of activation and hopping energy and large value of localization length in rGO-Si thin films help in efficient conduction of the photogenerated charge carriers (generated in Si micropillars) to the external circuit. To demonstrate the ballistic charge transfer through the rGO layers in the rGO-Si composites, magnetoresistance measurements (-7 T to 7 T at 300 and 5 K) are performed (inset of figure 7(a)). It is clear from the inset of figure 6(a) that at 300 K, the change in resistance with the applied magnetic field (-7 T to 7 T) is negligible. This is due to the dominance of Arrhenius conduction at high temperatures. Interestingly, there is ~2.5% negative magnetoresistance (MR) at 5 K (at 7 T). This electrical response of rGO-Si devices to an external magnetic field coupled with its optical characteristics will have potential implications in opto-spintronics.53 In a broader context, the MR effect can be attributed to the formation of charge puddles due to low activation energy (1.67 meV) in rGO-Si thin films.54,55 With the application of an external magnetic field (perpendicular to the thin film), the previously disconnected charge puddles merge54 thereby facilitating the formation of percolation pathways and reducing resistance (negative MR). The formation of negative MR (and its temperature dependence) is a clear indication of electronic charge transfer via rGO layers (as MR of intrinsic Si is always positive due to space-charge effect56). Large negative MR (~100%) have been reported in disordered structure of graphene57 as well as extremely small negative MR (~0.4%) have also been observed in single-layer pristine graphene.58 The negative MR in graphene-based 19 ACS Paragon Plus Environment


materials (GO and rGO) is attributed to the scattering of charge carriers at the sp2-sp3 boundaries. At low temperatures, the negative MR is more prominent (than at 300 K) due to variable range hopping conduction that causes the Coulomb blockade when the size of sp2 entities is small (small localization length). Therefore, with increase in disorder, the amplitude of negative MR also increases. Large values of negative MR in highly disordered GO and rGO structures are attributed to weak localization,57 which is not the case in the present situation as the localization length is large (17.58 m) for the case of rGO-Si thin films. The low values of MR (~2.5%) in the present case (rGO-Si thin films) are due to less defect concentration (sharp 2D Raman-active peak) and increased size of sp2 clusters (restoration of  continuum and increase in IG/ID in Raman spectra). All the above-mentioned factors favor a ballistic transport of the photogenerated charge carriers, which is a prerequisite for highly efficient photosensors. A plausible explanation of negative MR in rGO-Si thin films can be due to the (spin-orbit) interaction between the defects (on the basal plane of rGO) and localized carriers. According to the magnetic polaron model, the defects (vacancies, Stone-Wales) and functional groups (-OH, -COOH, etc.) in the rGO structure can induce magnetic moments.59,60 With the application of an external magnetic field, there is an alignment of the magnetic polarons, which increases the carrier hopping probability (negative MR). At low temperature (5 K), the phonon scattering is negligible and therefore the contribution of magnetic polaron-localized carrier conduction is more prominent (than at 300 K). Pulsed laser annealing is performed by irradiating thin films with laser of appropriate wavelength (193 nm), energy density (0.6-1.0 Jcm-2), and pulse duration (20 ns). The lasermaterial coupling causes absorption of laser photons (which get converted to heat thereafter). Upon incidence of the nanosecond laser, the thin film surface starts to melt slowly due to the


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sudden change in reflectivity during the phase transition. As the laser pulse terminates, the melt front recedes back to the surface (quenching). Therefore, PLA technique produces high temperature and melting, which assists in annealing out crystal imperfections (and forms highquality electronic materials and junctions).61,62 PLA can be carried out in air at 300 K since it is an ultrafast technique (completed in < 200 ns) and introduction of impurities from the atmosphere is minimized. The experimental measurements of diffusion coefficients in PLA indicate melting (of the thin film), which can introduce dopants beyond thermodynamic solubility limits.63,64 The underlying principle of PLA is undercooling, which develops tremendous amount of pressure thereby facilitating explosive recrystallization or formation of novel amorphous phases.16,65 In the present case, ArF nanosecond laser melts the composite layer of Si and C. The undercooling is the highest in the case of Si and lowest for C. The value of undercooling (∆𝑇) in Si is calculated to be ~300 K at 0.6 Jcm-2. In this highly supercooled state, the rate of nucleation (𝑟𝑛) is extremely high and is given using the equation: 𝑟𝑛 = 𝐶1exp ( ―𝐶2 (1 ― 𝑇/𝑇𝑚)2𝑇/𝑇𝑚

), where 𝐶1 and 𝐶2 are constants and 𝑇𝑚 denotes the melting point of Si.

The heat liberated from crystallization increases the temperature abruptly and causes explosive recrystallization.61 The Si micropillars are formed a result of explosive recrystallization, where the solidification velocity is calculated to be ~5msec-1. Since the Si micropillars and nanostructures are formed after the laser melting and subsequent quenching (of undercooled molten Si), electronic mixing of atomic orbitals occur thereby giving rise to new band structure (direct optical band gap transitions) in the Si nanostructures. In the case of C, melting and subsequent quenching (after PLA) occurs to form r-GO (no undercooling). It should be noted that in the case of amorphous C, low and high undercooling helps in the formation of diamond and Q-carbon, respectively.16 Therefore, PLA helps in the formation of Si micropillars and 21 ACS Paragon Plus Environment


nanostructures, which are well-connected to r-GO layers. This unique micro/nanostructure combination assists in formation of photogenerated charge carriers in the Si structures which are ballistically transported via r-GO layers to the external circuit. CONCLUSION A highly non-equilibrium technique (pulsed laser annealing) based on nanosecond laser melting and subsequent quenching of amorphous C and Si is used to fabricate highly sensitive large-area rGO-Si photosensors on c-sapphire. This ultrafast process (completed in < 200 ns) forms Si micropillars (~1 m) and nanoreceptors (~3 nm) on rGO layers, which lead to large photoresponsivity of rGO-Si devices. Raman spectrum indicates the presence of D, G, and 2D Raman-active vibrational modes in the PLA thin films, characteristics of high-quality rGO thin films. There occurs a red-shift of the G peak in rGO thin films as compared to that in GO which indicates removal of functional groups in rGO after the PLA induced reduction process. The nonappearance of asymmetry of the G peak in rGO structure (near 1600 cm-1) indicates the absence of graphite oxide in the rGO-Si composites. A higher intensity of the G peak as compared to D peak (IG/ID=1.13) and decrease in FWHM of the G peak in rGO depict a higher degree of reduction and lower degree of disorder (and distortion) in the sp2 structure. The intense and narrow 2D peak in rGO indicates a smaller number of defects (on the basal plane) in the rGO structure, which help in the ballistic conduction of photogenerated charge carriers. XPS spectra of rGO-Si indicate a low intensity of C=O bonding states (restoration of  electronic states), higher C/O ratio (4.06) as compared to rGO prepared using chemical and thermal processing routes, and lower intensity of the Si-O bonding states (as compared to Si-Si). All of these factors illustrate the formation of highly reduced rGO and minimal formation of SiO2 at the rGO-Si interface which will ultimately lead to better optoelectronic properties. Photoluminescence


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spectra of Si structures show a sharp peak centered at 903 nm which corresponds to radiative optical transition. The band structure analysis of PL spectrum illustrates a direct band gap (phononless optical transitions) in Si, which is caused due to quantum confinement effect and tensile strain (in the Si structures). This is essential for better optoelectronic activity in highfrequency applications. The formation of dome-shaped micropillars of Si and nanoreceptors of Si (~3 nm) on the micropillars enable better light absorption and creation of electron-hole pairs (photogenerated carriers). The SEM, EDX, and EBSD analyses indicate Si structures formed on well-connected rGO thin film, which allows ultrafast transfer of charge carriers via rGO to external circuit. The photosensors are Ohmic in nature and there are no S-shaped kinks (related to formation of SiO2) in the I-V measurements. There is a dramatic (exponential) decay of resistance when the white light source is turned ON and the resistance rises exponentially when the light source is turned OFF. The response and decay times are calculated to be 1.65 and 2.07 secs, respectively. A closer look at the decay profile indicates a faster decay region (𝜏𝑑 = 0.76 sec), which is followed by a longer decay (recombination) region. The stability and repeatability of the photosensing action in r-GO-Si devices are also observed for multiple cycles. To illustrate the transfer of the photogenerated charge carriers via the rGO thin films, temperature-dependent resistance and magnetoresistance characteristics of rGO-Si thin films are evaluated. The electrical characteristics show that the charge transport mechanism is dominated by band gap (Arrhenius conduction) and variable range hopping (ES-VRH) for T > 100 K and < 100 K, respectively. There is an improved electrical conductivity in rGO due to the creation and growth of new sp2 clusters at the expense of defects (-OH, -COOH, etc.) created on the basal plane of rGO.52 The formation of new sp2 clusters provides percolation paths between sp2 domains already present, which causes variable range hopping at low temperatures. In r-GO, the



localization length and hopping energy are calculated to be 17.58 m and 3.15 meV, respectively, by using ES-VRH. From Arrhenius conduction results, the activation energy in rGO thin films is determined to be 1.67 meV. These properties of rGO indicate efficient conduction of the photogenerated charge carriers to the external circuit. A low value of negative MR (~2.5%) in rGO-Si thin films indicates that the charge transfer is via rGO layers which are highly reduced and have less defects. The negative MR in rGO-Si thin films can be due to the (spin-orbit) interaction between the defects (on the basal plane of rGO) and localized carriers. The Ohmic nature of the fabricated rGO-Si photodetectors will enable its use in multifunctional solid-state devices, where miniaturization and high-frequency applications are highly desirable. The responsivity of these photosensors (fabricated using a facile route) are measured using white light (from halogen lamp) and can therefore be used for commercial purposes for superior photosensing applications. The rGO-Si photosensors can therefore have a myriad of applications ranging from radiation surveillance to biomedical imaging. METHODS Alternating layers of Si and amorphous carbon are deposited on c-sapphire substrates by using pulsed laser deposition (PLD) technique. The KrF excimer laser (pulse duration=25 ns, wavelength=248 nm) is used for pulsed laser deposition of the composite structure (thickness=500 nm) using energy density of 3.0-3.5 Jcm-2. The Si/amorphous carbon films are irradiated in air with ArF single laser pulse (pulse duration=20 ns, wavelength=193 nm) using a laser energy density of 0.6 Jcm-2. The ultrafast pulsed laser annealing technique melts amorphous carbon and Si from which rapid explosive recrystallization (of Si) occurs upon quenching to form dome-like structures of Si. The undercooling of amorphous carbon is decreased by increasing the sp2 carbon content in the as-deposited thin film. This leads to the formation of 24 ACS Paragon Plus Environment

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high-quality reduced graphene oxide structures on sapphire. Figure 1 shows the schematic of the synthesis of Si/rGO photosensors on c-sapphire substrates by using pulsed laser deposition and pulsed laser annealing. The formation of the highly effective photosensors is dependent on the melting and subsequent quenching (of molten C and Si) after the nanosecond laser is incident on the Si/C thin films. This melting and subsequent quenching depend on the laser energy density and thermal conductivities of the as-deposited layers and the substrate. The Si micropillars are formed a result of explosive recrystallization, where the solidification velocity is calculated to be ~5msec-1. Since the Si micropillars and nanostructures are formed after the laser melting and subsequent quenching (of undercooled molten Si), electronic mixing of atomic orbitals occurs thereby giving rise to new band structure (direct optical band gap transitions) in the Si nanostructures. Since the thermal conductivity of Si wafers is much higher than that of sapphire substrates, the formation of Si/rGO photosensors is not possible on Si wafers. The SLIM (simulation of laser interaction with materials) programming66 is used to simulate the laser-solid interactions which determine the formation of the novel photosensors. The SLIM calculations involve an accurate finite difference method to calculate the threshold energy density, melt depth, and solidification velocity of the pulsed laser annealed samples. The rGO-Si structures are characterized by Raman-PL spectroscopy, TOF-SIMS, XPS, high-resolution SEM, EDX, and EBSD in addition to temperature- and field-dependent resistance and photo sensing (resistance vs time) measurements. Alpha300 R – Superior Confocal Raman Imaging instrument is used for Raman spectroscopy measurements using 532 nm laser excitation source. For better spectral line resolution, 600 mm gratings are employed during the data collection procedure. Crystalline Si (having a strong peak at 520.6 cm-1) is used to calibrate the instrument before and after Raman spectrum acquisition process. High-resolution SEM and EDX measurements are carried out



using the field emission gun technique in FEI Verios 460L SEM. Time-of-Flight Secondary Ion Mass Spectrometer (TOF-SIMS) with a lateral resolution of

Reduced Graphene Oxide-Nanostructured Silicon Photosensors With

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