NARROWEST ARMCHAIR GRAPHENE NANORIBBON COMPOSITE FOR GAS SENSING AND FILTERING AND METHOD OF MANUFACTURING THEREOF

Abstract

Disclosed herein is a narrowest armchair graphene monolayer nanoribbon composite, and a method of its manufacturing. The method comprises steps of: forming (S1) an epitaxial graphene monolayer film in a low-pressure chemical vapor deposition (100); coating (S2) a polymethyl methacrylate (PMMA) coat on the graphene film using a spin coater (200); removing (S3) copper from the PMMA coated graphene film using ammonium persulfate; affixing (S4) the film onto a Si-SiO2 substrate followed by removing PMMA and drying; creating (S5) a photoresist layer on the Si-SiO2 substrate affixed graphene film and thereon nanoribbon pattern boundaries through extreme ultraviolet lithography (500); slicing (S6) the graphene film at its pattern boundaries through atomic layer etching (600) followed by removing impurities; and replacing (S7) two carbon atoms in each nanoribbon lattice with two dopant atoms of nitrogen (N) and manganese ejected in molecular beam epitaxy (700) to obtain the desired composite. The said nanoribbon composite exhibits improved gas sensing properties with air/gas filtration performance, and possesses biodegradability, environmental friendliness, non-toxic characteristics; thus, is found suitable for fabrication of gas filters and gas sensors. Fig. 2

Specification

Description:FIELD OF THE INVENTION
The present invention broadly relates to doped graphene nanoribbons. More specifically, the present invention relates to a narrowest armchair monolayer graphene nanoribbon composite doped with nitrogen and manganese, and a method of its manufacturing. The nanoribbon composite is used in fabrication/design of various gas filters and gas sensors.

BACKGROUND OF THE INVENTION
Many harmful gases such as nitrogen dioxide (NO2) gas are primarily emitted from various sources including but not limited to combustion processes, industrial operations, and other agricultural activities. It can lead to various health issues, including lung and cardiovascular diseases. To effectively control and monitor the hazardous NO2 molecules, there is a pressing need to delve into developing highly sensitive gas sensors. To address these critical issues, the researchers have explored various materials for sensing, including a variety of materials like metals (gold (Au), silver (Ag), and palladium (Pd)), elemental allotropes (borophene, mxene, phosphorene), and nanomaterial (graphene, graphene nanoribbon (GNR) and carbon nanotubes (CNT), etc.). However, excluding GNR, the other materials encounter severe limitations, such as less bandgap (EG) variation and low sensitivity to numerous pollutants. Therefore, the GNR is well suited for NO2 sensing among the defined sensing materials due to its exceptionally high surface-to-volume areas, reactive edges, and large carrier mobility. Hence, it is an optimal nanomaterial due to its versatility in bandgap engineering. This adaptability enhances their interaction with the NO2 molecules and their sensing efficiency.

Few investigation/simulations were conducted by researchers to understand effectiveness (properties) of the armchair graphene nanoribbon, however they all have one or more limitations. Initially, Sardana et al. investigated the modelling of doped and co-doped Armchair Graphene Nanoribbon (ArGNR) for the NO and NO2 sensing, but the mechanisms behind an enhanced adsorption in the co-doped systems are subjected to more investigation. Likewise, Verma et al. explored the adsorption of Specific PM2.5 Contaminants using Au and ArGNR Sensors. Nevertheless, the research overlooked the influence of doping on adsorption efficiency, and also, the influence of spin polarization on the electronic properties remains unexplored. Moreover, Tiwari et al. investigated the DFT (density functional theory) modelling on the B/N and B/P co-doped graphene for NH3 and NOx molecules. However, the investigation neglected the influence of co-doping in their structures for EG engineering. Following this, Hung et al. investigated the DFT modelling of the Tin (II) Sulphide (SnS) material for gas sensing but omitted the influence of adsorbate/adsorbent distance while exhibiting positive adsorption for the numerous molecules, including NO2 molecule. Furthermore, Gajjar et al. investigated the DFT modelling of doped graphene for molecules like NH3, SO2, and NO2 molecules. However, the analysis overlooked the surface reactivity and transport properties of the narrowest ArGNR device that can enhance the NO2 sensing.

Based on the previous research, the influence of properties like adsorption, desorption, surface reactivity, and bandgap on the sensitivity of the doped/undoped ArGNR remains unexplored. Further, the impact of spin polarization on EG engineering has yet to be extensively studied. Subsequently, to address these potential gaps, it necessities to examine the influence of spin polarization in the Hydrogen (H)-passivated doped/undoped narrowest ArGNR configurations for NO2 sensing. The reason behind the narrowest ArGNR is particularly compelling due to the strong quantum confinement effect where the electron wave functions are confined within a spatial region, resulting in the quantized energy levels. Furthermore, the contribution of the edge states to the overall electronic structure becomes more pronounced in these systems. This is primarily because the localized electronic states at the edges interact more strongly with the NO2 molecule as these edges are physically more accessible, thus playing a major role in the sensing mechanism.

Since, the existing graphene nanoribbons used in gas sensing/filtration technology have several limitations in terms of sensitivity, selectivity, miniaturization, energy efficiency, and safety, it necessitates to synthesize improved armchair graphene nanoribbon composite that can exhibit high absorption/desorption energy, low cohesive energy, and high sensitivity with filtration ability towards gaseous particulate matter, so that it can enhance operational efficiency while used in fabrication/designing of gas filters (for example face mask of chemical and medical industry) and gas sensors (for example gas sensors of power electronics). Moreover, it is desired to develop a narrowest armchair graphene nanoribbon composite doped with nitrogen and manganese, and a method of its manufacturing, which includes all the advantages of the conventional/existing techniques/methodologies and overcomes the deficiencies of such techniques/methodologies.


OBJECT OF THE INVENTION
It is an object of the present invention to investigate various armchair graphene nanoribbon applications in fabrication/designing of gas filters (to be used in chemical and health industry) and gas sensors (to be used in power electronics).

It is another object of the present invention to incorporate an optimal doping choice in armchair graphene nanoribbon synthesis to improve its various properties such as sensitivity, selectivity, miniaturization, energy efficiency, and safety in gas filtering/sensing application.

It is one more object of the present invention to synthesize an eco-friendly and non-toxic narrowest armchair graphene nanoribbon composite doped with manganese and one of group V elements, that can effectively sense and filter various gases (such as NO2) including PM2.5 contaminants present in the atmosphere.

It is a further object of the present invention to devise a robust, cost-effective, reliable, and energy-efficient method of manufacturing narrowest armchair monolayer graphene nanoribbon composite having dimensions of 4.35 Å length and 2.51Å and two carbon atoms are replaced with two dopant atoms of nitrogen and manganese.

SUMMARY OF THE INVENTION
The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. The summary's sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.

In one aspect, the present invention provides a method of manufacturing narrowest armchair monolayer graphene nanoribbon doped with manganese and nitrogen. The method comprises steps of: forming an epitaxial graphene monolayer film in a low-pressure chemical vapor deposition (LPCVD); coating a polymethyl methacrylate (PMMA) coat on the graphene film using a spin coater; removing copper from the PMMA coated graphene film using ammonium persulfate; affixing the film onto a Si-SiO2 substrate followed by removing PMMA and drying; creating a photoresist layer on the Si-SiO2 substrate affixed graphene film and thereon nanoribbon pattern boundaries through extreme ultraviolet (EUV) lithography; slicing the graphene film at its pattern boundaries through atomic layer etching (ALE) followed by removing impurities; and replacing two carbon atoms in each nanoribbon lattice with two dopant atoms of manganese and nitrogen ejected through molecular beam epitaxy (MBE) to obtain the desired composite. Preferably, the third and sixth carbon atoms (C3, C6) in the nanoribbon lattice are replaced with a nitrogen (N) atom and a manganese (Mn) atom, respectively. The proposed nanoribbon composite exhibits improved gas sensing properties with air/gas filtration performance, and possesses biodegradability, environmental friendliness, non-toxic characteristics; thus, is found suitable for fabrication of gas filters and gas sensors.

Other aspects, advantages, and salient features of the present invention will become apparent to those skilled in the art from the following detailed description, which delineate the present invention in different embodiments.

BRIEF DESCRIPTION OF DRAWINGS
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying figures.

Fig. 1 illustrates lattice (molecular) structure before and after doping of a narrowest armchair monolayer graphene nanoribbon.

Fig. 2 illustrates various equipment and materials used in manufacturing of the narrowest armchair monolayer graphene nanoribbon doped with nitrogen and manganese, in accordance with an embodiment of the present invention.

Fig. 3 illustrates various manufacturing method steps of doped narrowest armchair monolayer graphene nanoribbon, in accordance with an embodiment of the present invention.

Fig. 4 illustrates various components of low-pressure chemical vapor deposition (LPCVD) device used in growing armchair monolayer graphene film, in accordance with an embodiment of the present invention.
Fig. 5 illustrates various components of extreme ultraviolet (EUV) lithography device used for pattern boundaries creation on the graphene film, in accordance with an embodiment of the present invention.

Fig. 6 illustrates various components of atomic layer etching (ALE) device used for slicing of graphene film into narrowest graphene nanoribbons, in accordance with an embodiment of the present invention.

Fig. 7 illustrates various components of molecular beam epitaxy (MBE) device used for doping in the nanoribbon lattice to obtain the final composite, in accordance with an embodiment of the present invention.

Fig. 8 illustrates geometry (molecular structure) of different (doped and undoped) armchair graphene nanoribbon samples.

Fig. 9 illustrates adsorption energy graphical analysis of different ArGNR samples.

List of reference numerals
100 low-pressure chemical vapor deposition (LPCVD) device
102 gas chambers/cylinders
104 gas inlet
106 reaction chamber (quartz tube)
108 heaters
110 sample access door
112 vacuum generator/pump
114 cooler
116 vacuum sealed container
118 residue gas release outlet
200 spin coater
300 container containing copper foil removal solution
400 container containing organic solvents
500 extreme ultraviolet (EUV) lithography device
502 EUV plasma generator
504 illuminator optics
506 projection optics with six mirrors (M1-M6)
508 mask embedded with desired pattern
600 atomic layer etching (ALE) device
602 plasma gas
604 aluminium tube
606 ICP power house
608 matching unit
610 substrate table
612 turbo pump
614 oscilloscope
700 molecular beam epitaxy (MBE) device
702 nitrogen gas plasma generator
704 manganese loaded effusion cells coupled with heater
706 shutters
708 fluorescent screen for visualization
710 RHEED (Reflection High Energy Electron Diffraction) Gun for monitoring
712 quartz mask
714 cryopanels
716 sample holder
718 buffer chamber
GF graphene film
CF copper foil
Al aluminium substrate
PMMA polymethyl methacrylate coat/layer
Si/SiO2 silicon/silicon dioxide
PR photoresist layer
ArGNR armchair monolayer graphene nanoribbons
HMI Human machine interface

DETAILED DESCRIPTION OF THE INVENTION
Various embodiments described herein are intended only for illustrative purposes and subject to many variations. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but are intended to cover the application or implementation without departing from the scope of the present invention. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

The use of terms "including," "comprising," or "having" and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Further, the terms "an" and "a" herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Furthermore, the terms "at least one" and "one or more" herein are used to indicate one minimum number of components/features to be essentially present in the invention. The term 'doping' or 'doped' means substitution or replacement of carbon atom with dopant atom in graphene lattice.

In accordance with an embodiment of the present invention as shown in Fig 1, a narrowest armchair monolayer graphene nanoribbon (ArGNR) composite is depicted. The ArGNR is a single layer of carbon atoms arranged in a two-dimensional honeycomb lattice. The said ArGNR composite has dimensions of 4.35 Å length and 2.51Å width to accommodate six carbon atom (C1-C6) lattice, where two carbon atoms replaced with two dopant atoms of manganese (Mn) and nitrogen (N) in the nanoribbon lattice. The dopant atoms (Mn, N) in the nanoribbon lattice have concentrations of 16.67% and 16.67%, respectively (total concentration of dopants is about 33%). Preferably, the third and sixth carbon atoms (C3, C6) in the nanoribbon lattice are replaced with the nitrogen (N) and manganese (Mn) atoms, respectively. The width, structure, and edge configuration of the proposed nanoribbon are confirmed by various characterization techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy, etc.

In accordance with an embodiment of the present invention as shown in Fig. 2-3, the method for manufacturing narrowest armchair graphene nanoribbon composite is depicted. The method employs a low-pressure chemical vapor deposition (LPCVD) device (100), a spin coater (200), a container containing copper foil removal solution (300), a hot plate magnetic stirring container containing organic solvents (400), extreme ultraviolet (EUV) lithography device (500), an atomic layer etching (ALE) device (600), and a molecular beam epitaxy (MBE) device (700). The method comprises steps: forming (S1) an epitaxial graphene monolayer film (GF) on a copper foil (CF) in the LPCVD device (100); coating (S2) a PMMA coat on the graphene film (GF) using the spin coater (200); removing (S3) copper from the PMMA coated graphene film using ammonium persulfate in the container (300); affixing (S4) the PMMA coated graphene film onto a Si-SiO2 (silicon-silicone dioxide) substrate followed by removing PMMA and drying; creating (S5) a photoresist layer (PR) on the Si-SiO2 substrate affixed graphene film and thereon nanoribbon pattern boundaries using the EUV lithography (500); slicing (S6) the graphene film at its pattern boundaries using the ALE device (600) followed by removing impurities; replacing (S7) two carbon atoms in each nanoribbon lattice with two dopant atoms of nitrogen (N) and manganese (Mn) ejected in the MBE device (700) to obtain the desired composite.

In accordance with an embodiment of the present invention as shown in Fig. 4, the LPCVD device (100) used in the graphene film (GF) formation step (S1) comprises three gas chambers (102) containing argon (Ar) gas, hydrogen (H2) gas, acetylene (C2H2) gas; a gas inlet (104); reaction chamber (quartz tube) (106); heaters (108); a sample access door (110); a vacuum generator/pump (112); a cooler (water chiller) (114); a vacuum sealed container (116); a residue gas release outlet (214), and a human machine interface (HMI). At one end of the quartz tube (106), the gas inlet (104) is coupled to the gas chambers, and at other end of the quartz tube (106) the sample access door (110) is coupled to the dedicated vacuum sealed container (116). An aluminium substrate (Al) carrying a copper foil (CF) is used inside the reaction chamber for graphene formation. The copper (CF) functions as a catalyst in facilitating graphene growth, where the alumina (Al) acts as a protective thermal barrier diminishing the possibility of thermal strain and deformation of the CF during the graphene growth at elevated temperatures. The substrates are thoroughly cleaned using appropriate cleaning procedures, including sonication in solvents and chemical treatments, and a magnetic stirrer with a hot plate, to remove contaminants and native oxides that could adversely affect the growth process.

For graphene film (GF) formation, the CF is placed atop the Al substrate in heating zone of the reaction chamber through its sample access door. To ensure an inert atmosphere (evacuation until reaching a pressure of 160 mTorr) the vacuum is created inside the reaction chamber using the vacuum pump. The heaters are tuned on to heat at a rate of 20℃ per minute, where the Cu and Al are annealed upto a temperature of 1000℃ for a period upto 45 minutes; and simultaneously the hydrogen (H2), the argon (Ar), and the acetylene (C2H2) gases are injected into the reaction chamber from the corresponding gas cylinders. If the temperature inside the reaction chamber exceeds the defined temperature range, the cooler is used to keep it under control. The mass flow control (MFC) of the gases can be controlled very precisely through the HMI. The hydrogen (H2), the argon (Ar), and the acetylene (C2H2) (carbon source) gases are injected at mass flow rate of 30-40sccm, 145-155sccm, and 2-10sccm, respectively. Preferably, the mass flow rate of hydrogen (H2) and argon (Ar) gases maintained at 35sccm and 150sccm, respectively.

Under controlled conditions and elevated temperatures, the precursor gas (C2H2) decomposes on the surface of CF forming a graphene film/layer. Once the graphene monolayer is formed on the CF, it is allowed to cool therein or transferred (with the CF) into the vacuum sealed container for proper cooling (along with prevention of reaction with environment), as it is crucial to stabilize the grown graphene structure and prevent any unwanted changes that can occur during rapid temperature changes. The cooling rate is controlled to ensure preservation of the desired graphene properties. Further, the monolayer structure of the grown graphene sample can be confirmed by Raman spectroscopy.

In accordance with an embodiment of the present invention, the PMMA coating step (S2) comprises using the spin coater (300) to uniformly apply/spread a thin layer of the PMMA coat on the graphene film (GF) placed on the copper foil (CF) by centrifugal force, and allowed to be dried.

In accordance with an embodiment of the present invention, the copper removal step (S3) comprises immersing the film in the container (300) containing a chemical etchant such as (1 molar) ammonium persulfate; rinsing the film with deionized water, and then drying the rinsed film in in a controlled environment such as a nitrogen-filled glovebox or under a heat lamp. The etchant reacts with the copper, dissolving it away, where The PMMA layer protects the graphene from being etched. The deionized water removes residual etchant.

In accordance with an embodiment of the present invention, the Si-SiO2 substrate affixing step (S4) comprises washing away the PMMA coat with an organic solvent such as acetone under the hot plate magnetic stirring (400). The Si-SiO2 substrate affixed graphene film is further rinsed with more solvent to remove any remaining PMMA contaminants, then it is dried.

In accordance with an embodiment of the present invention, before pattern creation in the step (S5) the spin coater (300) is used to uniformly apply/spread the photoresist layer (PR) such as poly hydroxy styrene onto the Si-SiO2 substrate affixed graphene film (GF), where the substrate is rotated at high speeds, and such rotation generates a centrifugal force that ensures an even distribution of the resist across the GF surface. The final thickness of the resist layer can be finely controlled by adjusting the spin speed and duration, relying on fluid dynamics principles, where the viscosity of the resist and the rotation speed play significant roles in determining the uniformity and thickness.

In accordance with an embodiment of the present invention, as shown in Fig. 5, the EUV lithography device (500) used in the pattern creation step (S5) comprises a EUV plasma generator (502), an illuminator optics assembly (504), a projection optics with six mirrors (M1-M6) (506), and a reflective mask embedded with desired pattern (508). The pattern has a dimension equivalent to nanoribbon lattice (molecular structure). The EUV lithography technology uses a plasma source, typically treated by hitting a target material (like tin) with high-energy light amplification by stimulated emission of radiations (LASER) that produce EUV light (plasma) at the desired wavelength (preferably ultraviolet light of 13.5 nm wavelength) in a vacuum environment. The mask is designed with extreme precision to ensure that the desired pattern can be transferred accurately on the GF surface. Subsequently, the mask is designed in such a way with intricate patterns that represent the desired features to be fabricated on the GF surface, including alternate opaque light blocking and transparent light-reflecting layer. This mask is typically built of substrate material like silicon coated with a highly reflected layer. The step (S5) includes exposing the photoresist coated graphene film with Si-SiO2 substrate to the EUV light that is generated by the EUV plasma source, and directed onto the mask using the illuminator optics, which ensure uniform illumination across the mask. Subsequently, as the EUV light strikes the mask, the reflective region reflects the light toward the GF surface, while the opaque region absorbs it, preventing light from reaching the underlying photoresist on the GF surface. Consequently, the reflected light from the mask is then captured by the projection mirror optics (M1-M6). These are designed to focus and project the reflected light on the wafer. After that, the projection optics can also magnify the image, allowing for the fabrication of features at a scale that may differ from the original mask dimensions. The photoresist is when exposed to EUV radiation, it activates the photo acid generator (PAG), commonly triphenyl sulfonium salts, within the resist. The PAG generates a strong acid in the exposed regions, altering the solubility of the resist and effectively setting the stage for the nanoribbon pattern. After the exposure, a post-exposure bake is performed which allows generating acid to diffuse throughout the resist, catalysing the further chemical reaction that enhances the contrast between exposed and unexposed areas. This increased contrast is vital for the precise patterning required in the GF surface.

In accordance with an embodiment of the present invention, after pattern creation in the step (S5), the resist is treated with an appropriate solution or a developer specific to the resist chemistry. Preferably, the unexposed regions of the graphene film are washed away with an alkaline solution such as tetramethylammonium hydroxide (TMAH) solution to retain only the defined patterns. This alkaline solution selectively removes the unexposed areas of the positive resist, allowing the exposed regions to remain intact due to their reduced solubility. The concentration of TMAH can vary, typically ranging from 0.26% to 2.38%, depending on the specific resist and desired development characteristics. After the lithography process the high-resolution atomic force microscopy is utilized for the verification of dimension and quality of the narrowest graphene nanoribbons.

In accordance with an embodiment of the present invention, as shown in Fig. 6, the ALE device (600) used in the graphene film slicing step (S6) comprises a plasma gas generator (602), an alumina tube covered with copper coil (604), an ICP power house (606), a current/voltage matching unit (608), a substrate table (610) under the alumina tube (604), a turbo pump (612) for creating vacuum, and an oscilloscope (614) with high voltage probe and low-frequency waveform biasing features. The patterns are just on surface of the photoresist layer (PR), and it doesn't change the material (GF with Si-SiO2 substrate) affixed beneath it. The etching is necessary because it removes material from the material underneath the photoresist layer. The etching gives physical shapes and features, and converts each pattern into a real 3D ArGNR slice. The Atomic Layer Etching (ALE) is a precision etching technique that operates on an atomic scale, enabling controlled material removal layer by layer, which is crucial for fabricating narrowest armchair graphene nanoribbon (ArGNR) requiring dimensions in nanometre range. The reactive gases (chlorine or fluorine), are introduced in plasma form towards the patterned GF substrate placed on the substrate table. The copper coil around the alumina tube helps in sustaining the plasma and ensuring uniform distribution of reactive species across the GF substrate. The alumina tube is a chamber lining being resistant to chemical reactions with the plasma and etching gases, maintaining the integrity of the system. The turbo pump is used to maintain a vacuum environment in the etching chamber, crucial for reducing contamination and facilitating the transport of reactive gases to the GF substrate. The oscilloscope is used to monitor and analyze the electrical signals and waveforms generated during the ALE process, providing feedback on system performance and etching conditions. The ICP power house supplies the power necessary to sustain the plasma, and operates at high frequencies, enhancing ionization and enabling a more uniform plasma generation. The matching unit ensures efficient power transfer from the power supply to the plasma chamber, optimizing the energy input and improving the overall etching efficiency. A RFEA (Retarding Field Energy Analyzer) may be used in the ALE device to measure the energy and flux of ions reaching the substrate, allowing for precise control over the etching step and ensuring the desired ion energy is maintained.

In accordance with an embodiment of the present invention, before slicing/etching in the step (S6) the spin coater (200) is used to uniformly spread/apply a PMMA layer on the graphene film. After slicing/etching in the step (S6), the PMMA and photoresist impurities/contaminants are washed away with a suitable organic solvent under hot plate magnetic stirring. The organic solvents may include acetone, ethyl lactate, isopropanol for complete removal of PMMA, and the tetramethylammonium hydroxide (TMAH) solution is used for complete removal of photoresist layer. Then, the ArGNR slices are dried using any critical point dryer. It is also essential to inspect the cleaned and dried nanoribbon sample to ensure that it is free of contaminants, residues, and other defects, which can be done through optical microscopy, scanning electron microscopy (SEM), or similar testing equipment.

In accordance with an embodiment of the present invention as shown in Fig. 7, the MBE reactor/device (700) used in the carbon replacement step (S7) comprises a nitrogen plasma generator (702), manganese powder loaded effusion cells (704), shutters (706), fluorescent screen (708), a RHEED (Reflection High Energy Electron Diffraction) Gun (710), a quartz mask (712), cryopanels (714), a sample holder (716), and a buffer chamber (718). The whole setup is arranged in a controlled ultra-high vacuum chamber. The effusion cells are formed of tantalum or tungsten like material, and loaded with the dopant containing compounds (manganese powder). The RHEED gun helps monitor the surface structure and allows for adjustments to be made during deposition, ensuring that the doping occurs at the right stages and uniformity is maintained. The fluorescent screen provides visual insights into the surface quality and the effectiveness of the doping. The cryopanels help to maintain low temperatures in parts of the chamber, reducing the likelihood of unwanted reactions or contamination and enhancing the quality of the deposited layers. The buffer chamber acts as a transition zone between the effusion cells and the growth chamber, and helps to maintain a stable environment, minimizing contamination and ensuring that the flux of dopants is uniform before reaching the substrate. The mask has two holes corresponding to the third and sixth carbon atoms (C3, C6) of the nanoribbon lattice. The mask is appropriately positioned in front of the ArGNR slice that is mounted in the sample holder. There may be six shutters corresponding to six carbon atoms in the nanoribbons lattice, and these shutters are in front of the mask. The nitrogen plasma generator and the effusion cell are provided with heaters which are turned on generating the respective dopant molecular beams that are self-propelled (due to principles of molecular effusion and vacuum environment) towards the ArGNR slice surface passing through the holes of the mask. The shutters are used to control exposure of the ArGNR molecular structure to the dopant flux (N, Mn). By opening and closing the shutter at precise intervals, the accurate layering and doping profiles can be achieved. Preferably, in the ArGNR lattice C3 atom is replaced with N, and C6 is replaced with Mn. The sample holder has a heater which can help in annealing the ArGNR after the dopant atoms get incorporated in its lattice structure to obtain the final N-Mn doped narrowest armchair monolayer graphene nanoribbon composite that is found to exhibit improved electronic properties, thus stands suitable for the fabrication of gas sensors and gas filters.

Experimental Analysis

The industry-standard Virtual Nano Lab system (QuantumATK) is used to analyze the atomic structure of doped/undoped ArGNR samples using the DFT (Density Function Theory) computation. A screened Generalized Gradient Approximation (SGGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional approach is used in the LCAO (Linear combination of atomic orbitals) calculator for attaining an improved accuracy of DFT. Moreover, the Fritz-Haber-Institute (FHI) pseudopotential in conjunction with a double zeta polarized (DZP) basis set is utilized, customizing pseudopotentials to replace tightly bound core electrons, thereby enhancing the precision of the structural computations.

Multiple doped/undoped ArGNR composite samples are synthesized through the QuantumATK, and their molecular structures (geometrical dimensions) are shown in Fig. 8. Five samples such as; Sample-1: Pristine (undoped), Sample-2: Mn-doped at C6 site, Sample-3: Mn doped at C6 site with N-doped at C3 site, Sample-4: Mn-doped at C6 site with P-doped at C3 site, and Sample-5: Mn-doped at C6 site with As-doped at C3 site, are selected for investigation. All the samples have equal lengths and breadths of 4.35Å and 2.51Å, respectively. The C-C bond length of 1.42Å characterizes the structure, featuring H-passivated edges with a C-H bond length of 1.09Å. The technical effects of all samples are shown in Table 1.
Table 1: Technical effects of all samples
Parameters Sample-1 Sample-2 Sample-3 Sample-4 Sample-5
Configurations Pristine ArGNR Mn doped ArGNR Mn-N co-doped ArGNR Mn-P co-doped ArGNR Mn-As co-doped ArGNR
Atoms 6 6 6 6 6
Spin effect Yes Yes Yes Yes Yes
Adsorption energy (Eads) in eV -0.61 -2.92 -2.67 -1.15 -1.85
Bandgap energy (EG) in eV (B.A.) 1.72 0.85 1.15 1.38 0.92
Bandgap energy (EG) EG in eV (A.A.) 0.1 0.71 0.96 0.96 1.04
EB in eV - 157.39 308.47 310.57 312.13
Desorption (B.A.) in eV 3.30 1.90 1.94 1.95 2.89
Desorption (A.A.) in eV 2.04 2.95 2.71 2.51 2.80
*B.A.- before adsorption; A.A.- after adsorption

Different widths of the pristine ArGNR structure are optimized without the presence of the NO2 molecule, and the total energy of each configuration are calculated. After the optimization, the NO2 molecule is purged into the ArGNR configurations, and the various electronic properties, including the total energy and the band structure, are calculated before and after the adsorption of the NO2 molecule. For all the computations, the k-point sampling of 1×1×99 k-point grid is utilized in the x, y, and z-axes, respectively, with a broadening of 300 K and a density mesh cut-off of approximately 75 Hartree. To ensure numerical stability, a series of self-consistent calculations are performed with fixed stress and force tolerances set at 0.05 eV/Å3 and 0.05 eV, respectively. Finally, the adsorption energy (Eads) is calculated by comparing the individual energies of the NO2 contaminant and the pristine ArGNR with and without the adsorption of NO2 molecules, respectively using equation 1.
equation [1]
where, EArGNR/NO2, EArGNR, and ENO2 stand for the aggregate enthalpy energy of the ArGNR sample with the adsorption of the NO2 molecule, energy of the ArGNR sample and the NO2 molecule, respectively. The results demonstrate that the width 3 ArGNR has a better proportion of edge atoms than the bulk atoms. In addition, it has optimal Eads compared to the different widths of the ArGNR. Notably, following the NO2 adsorption, it introduces additional states near the fermi levels, leading to a significant reduction in the EG in width 3, depicted in Table 2.

Table 2: ADSORPTION IN SITE C6 IN DIFFERENT WIDTH VARIATIONS
S.NO. Polarised (at site C6)
WIDTH VARIATION ADSORPTION
ENERGY
(eV) BAND GAP (Eg)(eV) % BAND GAP (Eg)(eV)
Eg
(Before Adsorption) Eg
(After
Adsorption)
1. Width-3 -0.40 1.72 0.68 60.47%
2. Width-4 -0.31 2.55 1.31 48.63%
3. Width-5 -0.41 0.49 0.26 46.94%
4. Width-6 -0.39 0.96 0.53 44.79%
5. Width-7 -0.36 1.69 0.87 48.52%
6. Width-8 -0.42 0.38 0.21 44.74%
7. Width-9 -0.39 0.65 0.30 53.84%

The variation in the EG is from a non-zero semiconducting bandgap to a metallic bandgap, a critical characteristic for gas sensing. However, wider ribbons' edge effects are diminished as the bulk properties dominate. However, despite the large value of Eads, their % EG is less compared to the narrow ArGNR.

The width-3 pristine ArGNR configurations exhibit six carbon (C) sites denoted as C1, C2, C3, C4, C5, and C6. Among these sites, the C2, C3, C5, and C6 are bonded with H-passivation, while C1 and C4 exhibit non-passivated edges. Subsequently, the NO2 molecule is purged at each C site with an adsorption height of 1.5Å for the determination of an optimal adsorption site. After then using equation 1, the Eads of each site are calculated, illustrated in Table 3.

Table 3. IDENTIFICATION OF OPTIMAL SITE
Adsorption sites height 1.50Å
Site E_ads (eV) EG (B.A) (eV) EG (A.A) (eV)
C1 -0.32 1.72 0.87
C2 -0.30 1.72 0.75
C3 -0.39 1.72 0.70
C4 -0.32 1.72 0.85
C5 -0.30 1.72 0.76
C6 -0.40 1.72 0.68

It has been observed that the C6 site exhibits the highest Eads, indicating its superior adsorption capability compared to other C sites of the ArGNR. Consequently, in the context of single doping, the C6 site is deemed suitable; however, for co-doping, further analysis beyond C6 is necessary. Among the available sites, including C1, C2, C3, C4, and C5, the C3 site is chosen due to its comparable Eads with the C6 site. Consequently, it is concluded that the C6 site is optimal for single doping, while both C3 and C6 sites are identified as an optimal site for co-doping. The rationale behind this is that the presence of H-passivated sites creates favorable conditions for the NO2 adsorption by forming stable bonds with the C atoms, enhancing the interactions and leading to stronger adsorption compared to the other C sites. The further discussion highlights the importance of considering EG variation for identifying an optimal site. Prior to NO2 adsorption, both the C3 and C6 depict an EG of 1.72eV, indicative of its semiconducting nature. Following the NO2 adsorption, the EG is decreased to 0.68 eV, while C3 is displayed an EG of 0.70 eV, indicating high sensitivity.

The binding energy (EB) is defined as the energy required for implanting and substituting the doped atom into the lattice site of the ArGNR configurations expressed in equation 2
equation 2
where Edoped /ArGNR, EArGNR, EMn, EN, EP, and EAs are the total energy of undoped/doped ArGNR and the energy of Mn, N, P, and As elements, respectively. Initially, when Mn is substituted at the C6 lattice site of the ArGNR, it requires less enthalpy energy for implanting in the ArGNR site compared to the co-doped ArGNR. This is because substituting a single dopant in the lattice results in severely less heat dissipation, which results in less enthalpy value. However, substituting Mn at the C6 site with successive substitution of N, P, and As at the C3 site requires more energy to implant in the lattice site of the ArGNR, resulting in a large enthalpy and EB value. Additionally, the variation in the atomic radius of group Vth elements (N: 56pm, P: 98pm, and As: 114pm) increases, further contributing to an elevated EB, illustrated in Table 1.

The increase in EB results from the significant impact of varying the atomic radii, which leads to alteration in the bond length within the co-doped ArGNR. Further, this alteration in the bond length accentuates the surface instability in the co-doped ArGNR compared to Mn-doped ArGNR. Consequently, this surface instability induces an outward force near the co-doped ArGNR atoms, thereby augmenting the EB relative to that observed in the Mn-doped ArGNR.

Referring to Fig. 9 and Table 1, the adsorption and desorption behaviour of all samples are tested. Initially, the sample 1 exhibits physisorption at the C6 site. This physisorption arises due to the interaction of the NO2 molecules with the delocalized π-electrons at the C6 site of the ArGNR lattice, resulting in the weak Van der Waals forces. However, the introduction of the dopants leads to an improvement in the Eads. Specifically, Sample-2 results in a substantial increase in the Eads, indicating chemisorption. This increase in adsorption is attributed due to the presence of partially filled d-orbitals in the Mn atom. Further, for the co-doping analysis, the Mn is substituted with group V elements (N, P, and As) at the C6 and C3 sites.

In a precise manner, Sample-3 results in the substantial chemisorption. This increased adsorption can be attributed to N, which is more electronegative and has a smaller atomic size that can introduce additional electronic states near the fermi level and modify the local electronic structure around the Mn atom. Conversely, Sample-4 and Sample-5 exhibit less chemisorption, respectively, than Sample-2 and Sample-3. In this case, the P and As dopants do not introduce significant electronic modification near the Mn adsorption site. Their larger atomic sizes and lower electronegativity (χ) values could lead to a weaker interaction with the neighboring C atoms and less pronounced changes in the electronic structure. In the context of NO2 desorption, a significant adsorbate-to-adsorbent distance after the adsorption phase can be considered a positive sign of desorption. This means the NO2 molecules interact with the adsorbent and are more likely to be desorbed from its surface. This means samples 2, 3, and 4 give better desorption than samples 1 and 5.

The Bandgap (EG) variation of doped/undoped ArGNR before and after the adsorption of the NO2 molecule is investigated. Initially, the analysis focuses on Sample-1, where the small variation in the bond length leads to a significant % EG variation, with a decrease in the EG post NO2 adsorption, delineated in Table 1. The reduction in the EG is attributed to spin polarization effects induced by the interaction between ArGNR and the NO2 molecule. Consequently, in Sample-2, Sample-3 and Sample-4, there is a notable decrease in the EG is evident. The reduction in EG suggests a significant shift in the electronic structure, likely attributed to spin polarization effects. Further, the adsorption of the NO2 molecule induces spin polarization and consequently alters the electronic properties, leading to a decrease in the EG. Furthermore, in Sample-5, subsequent to the optimization process, there is a noteworthy increase in EG following the adsorption of the NO2 molecule. It is due to significant changes in the electronic structure, including the shifting of electronic states away from the fermi level (EF). This shift leads to an increase in the effective EG of the material, indicating decreased conductivity, depicted in Table 1. Comparing to all the samples there is noteworthy bandgap variation in Sample-3 and Sample-4.

The molecular configuration and performance metrics of the Mn-N co-doped narrowest ArGNR composite sample (present invention) are compared with that of the conventional graphene nanoribbon samples as shown in Table 4.
Table 4
Parameters of ArGNR
Deji et al.
Solanki et al.
Y. Liu et al.
Salih, et al.
Salih et al.
Esrafili et al. Present Invention
Configurations BCP2-ArGNR ArGNR CuO-PtSSe MoS2 ZGNR Graphene Mn-N co-doped ArGNR composite
Total Atoms 14 28 48 5×5 32 32 (4×4) 6
Spin effect No No No No No No Yes
Adsorption energy (EG) in eV -4.57 -4 -2.12 -2.60 −0.225 -0.61 -2.67
Bandgap energy (EG) in eV (B.A.) No bandgap 0.61 1.05 0 0 0 1.15
Bandgap energy (EG) in eV (A.A.) 0.17 1.05 1.43 0 0 0.57 0.96

It is observed that the proposed ArGNR composite exhibits spin effect, significant EG variations, optimal bandgap transitioning from a semiconducting to a semi-metallic state, indicating heightened sensitivity than the conventional graphene nanoribbons. Thus, the proposed ArGNR composite can be advantageous for applications involving gas sensing and molecule adsorption.

The foregoing descriptions of exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiment was chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable the persons skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions, substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but is intended to cover the application or implementation without departing from the scope of the claims of the present invention. , Claims:We claim:

1. A method of manufacturing armchair graphene nanoribbon composite, the method comprises steps of:
forming (S1) an epitaxial graphene monolayer film on a copper foil placed atop an aluminium substrate in a reaction chamber of a low-pressure chemical vapor deposition (LPCVD) device (100);
coating (S2) a polymethyl methacrylate (PMMA) coat on the graphene film formed on the copper foil through a spin coater (200);
removing (S3) the copper foil from the PMMA coated graphene film through immersion in ammonium persulfate (300);
affixing (S4) the PMMA coated graphene film onto a silicon-silicone dioxide (Si-SiO2) substrate followed by removing PMMA and drying;
creating (S5) a photoresist layer on the Si-SiO2 substrate affixed graphene film and thereon nanoribbon pattern boundaries through extreme ultraviolet (EUV) lithography device (500), wherein each of the nanoribbon patterns has 4.35 Å length and 2.51Å width accommodating six carbon (C1-C6) atoms;
slicing (S6) the graphene film at its pattern boundaries through atomic layer etching (ALE) device (600) followed by removing impurities to obtain armchair graphene nanoribbons; and
replacing (S7) two carbon atoms in each nanoribbon lattice with two dopant atoms of nitrogen (N) and manganese (Mn) ejected through molecular beam epitaxy (MBE) device (700) to obtain the desired composite.

2. The method as claimed in claim 1, wherein the graphene film forming step (S1) comprises:
creating vacuum with pressure upto 160 Millitorr in the reaction chamber through a vacuum pump;
annealing the substrate through a heater at a temperature upto 1000℃ for a period upto 45 minutes;
injecting hydrogen (H2), argon (Ar), and acetylene (C2H2) from corresponding gas chambers into the reaction chamber at mass flow rate of 30-40sccm, 145-155sccm, and 2-10sccm, respectively; and
cooling the substrate after growth of the graphene monolayer film on the copper foil.

3. The method as claimed in claim 1, wherein the copper removing step (S3) comprises rinsing the PMMA coated graphene film with deionized water, and drying the rinsed graphene monolayer in a nitrogen-filled glovebox or under a heat lamp.

4. The method as claimed in claim 1, wherein the Si-SiO2 affixing step (S4) comprises washing away the PMMA coat with acetone.

5. The method as claimed in claim 1, wherein the pattern creating step (S5) comprises:
applying poly hydroxy styrene photoresist layer onto the Si-SiO2 substrate affixed graphene film through the spin coater (200);
exposing the graphene film under ultraviolet light of 13.5 nm passing through a reflective mask with optical mirror assembly fitted in a vacuum chamber to mark the pattern boundaries; and
washing away unexposed regions of the graphene film with tetramethylammonium hydroxide (TMAH) solution to retain the defined patterns.

6. The method as claimed in claim 1, wherein the graphene film slicing step (S6) comprises:
applying a PMMA layer on the graphene film through the spin coater (200) before etching; and
washing away the PMMA and photoresist impurities after etching with acetone, ethyl lactate, isopropanol, and tetramethylammonium hydroxide (TMAH) solution under hot plate magnetic stirring; and
drying the armchair graphene nanoribbons through a critical point dryer.

7. The method as claimed in claim 1, wherein the carbon atom replacing step (S7) comprises introducing the dopant atoms (N, Mn) to pass controllably through holes formed on a quartz mask positioned before the armchair graphene nanoribbon in a sample holder of the MBE device (700), wherein the holes of the mask are aligned to third and sixth carbon atoms (C3, C6) of the nanoribbon lattice.

8. A armchair graphene nanoribbon composite, comprises:
an armchair graphene nanoribbon having dimensions of 4.35 Å length and 2.51Å width to accommodate six carbon atom lattice; and
two carbon atoms replaced with two dopant atoms of nitrogen (N) and manganese (Mn) in the nanoribbon lattice.

9. The armchair graphene nanoribbon composite as claimed in claim 8, wherein the dopant atoms (N, Mn) in the nanoribbon lattice have concentrations of 16.67% and 16.67%, respectively.

10. The armchair graphene nanoribbon composite as claimed in claim 8, wherein third and sixth carbon atoms (C3, C6) in the nanoribbon lattice are replaced with nitrogen (N) and manganese (Mn), respectively.

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