The mid-wavelength region from 3 μm to 5 μm portion of infrared radiation band（IR），is an atmospheric window for IR seekers to detect the infrared signature of object^［1］. It gives a rise to a great deal of application such as night vision，thermography，remote control，molecular spectroscopy，climatology and meteorology，etc^［1］.An infrared signature depends on many factors，including the shape and size of the object^［2］，temperature^［3］，and emissivity，reflection of external sources（earthshine，sunshine，skyshine）from the object's surface^［4］，the background against which it is viewed^［5］ and the waveband of the detecting sensor. Due to the requirement of national defense and border security，any technique reducing infrared signature of significant objects is able to avoid the attack by infrared guided weapons and infrared surveillance sensors^［6-7］，and thus increases the object overall survivability. Aforementioned demands stimulates continuous interests in seeking for mid-wavelength infrared absorbing materials.

Two-dimensional transition metal carbides and nitrides（MXenes）have shown diverse light-matter interactions in a broad frequency range^［8］，owing to their highly anisotropic electronic and optical properties，easy-to-functionalize surfaces，and high density of states at the Fermi level^［8］. For instance，The absorption rate of the 15 μm thick Ti₃C₂T_x film in the visible light range（380~780 nm）is around 90%，while it decreases rapidly in the near-infrared range（1~3 μm），reaching a minimum of ~10% in the mid-infrared range^［9］. Li placed thin film materials such as MXene，graphene，graphene oxide，montmorillonite and stainless steel on a high-temperature substrate（508°C），and the measured radiation temperatures were 181°C，223°C，294°C，421°C and 159°C respectively^［10］. As such，Ti₃C₂T_x exhibits excellent low infrared radiation characteristics. Deng combined the concept of multi-color printing and printed Ti₃C₂T_x solutions with different infrared emissivities into composite anti-counterfeiting patterns，so that the infrared anti-counterfeiting patterns can be identified and decrypted with the help of a thermal image camera^［11］. In the solid solution of MXene，precise control of infrared emissivity from low to high can be through changes in composition and atomic structure. In addition，utilizing the electrochemical properties of MXene，the electrochemically driven infrared emissivity can be controlled by applying an external electric field^［12］. This shows that MXene has huge application potential in the fields of optical communication，imaging，and thermal management in the infrared band，and that the surface plasmons of MXene can be controlled through MXene components，surface groups，and film thickness. Utilizing these properties of MXene for metasurface design promises new breakthroughs. However，there are currently few reports on the basic dielectric properties of different types of MXenes in the infrared band，and further exploration is urgently needed.

In this work，we present a chemical method to deform the MXene flakes locally by grafting two types of alkane octane（C₈H₁₈）or dodecane（C₁₂H₂₆）on the lattice-site of terminated functional group of Ti₃C₂T_x MXene. Then，MXene thin-film is achieved by the drop-cast process. The Atomic force microwscopy（AFM）in combination with transmission electronic microscopy（TEM）and scanning electronic microscopy（SEM）reveal the morphological change of Ti₃C₂T_x MXene. The mid-wavelength IR absorption of Ti₃C₂T_x MXene and alkane grafted ones are tested by Fourier transform infrared spectroscopy（FT-IR）. Correspondingly，IR absorption enhancement owing to flake-curvature of alkane grafted Ti₃C₂T_x MXene thin-film are discussed.

The Ti₃C₂T_x MXene is synthesized by a top-down method of chemical exfoliation. All the chemicals are purchased from Sigma-Aldrich^® Lab & Production Materials of Merck Ltd. Figure 1 shows the schematic diagram of the flakes of Ti₃C₂T_x，Ti₃C₂T_x-C₈H₁₈，Ti₃C₂T_x-C₁₂H₂₆ MXene. First，the 0.5 g of Ti₃AlC₂ MAX phase precursor was added into a mixture of deionized（DI）water（13.5 mL）/ HF（3 mL），and stirred at room temperature for 24 hours. The sediment is washed with DI water several times until the pH value reached neutral，which is then added into 15 mL DI water containing 1.5 g of LiCl under vigorous stirring at room temperature for 12 hours. The monolayer Ti₃C₂T_x suspension is washed with DI water several times and collected by centrifugation at 3 500 rpm for 5 min. Second，centrifuged the prepared single-layer or few-layer Ti₃C₂T_x solution at high speed and took out 1g sediment after the solvent was completely removed. Separately added DI water（10 ml），ethanol（160 ml），and aqueous ammonia（15 ml），the mixed solution was fully stirred for 24 hours. Then，1 ml of（octyl）-triethoxysiloxane（OTES）was introduced into the above solution and stirred vigorously for 24 h again. After the reaction was finished，de-ionized water and ethanol were used for centrifugal cleaning in turn to obtain hydrophobic Ti₃C₂T_x-C₈H₁₈ MXene flakes. Third，centrifuged the prepared mono-layer Ti₃C₂T_x solution at high speed and took out 1g sediment after the solvent was completely removed. Separately added DI water（10 ml），ethanol（160 ml），andaqueous ammonia（15 ml），the mixed solution was stirred for 24 hours. Then，1 ml of（dodecyl）-triethoxysiloxane（DCTES）was introduced into the above solution and stirred vigorously for 24 hours again. After the reaction was finished，de-ionized water and ethanol were used for centrifugal cleaning in turn to obtain hydrophobic Ti₃C₂T_x-C₁₂H₂₆ flakes.

The MXene thin films are achieved by drop-cast and spin-coating process. The polyimide substrate was pre-ultrasonically cleaned with ethanol and de-ionized water for 20 min. Aforementioned MXene flakes are dripped onto a 75 μm thick polyimide（Du Pont^® Kapton^TM HN）membrane of 20×15 mm area and spin-coating for 30 seconds at 500 rmp first followed by 15 seconds at 1500 rmp later. The as-fabricated MXene thin-film is shown in Fig. 1（b）. The MXene specimen is in the x-y plane，and the incident IR radiation is along the z axis. The transmittance spectra of the MXene were measured by a FT-IR system（JASCO FT/IR-660 plus）with resolution is 1 cm^-1 and the scanning range is from 2.5 μm to 4.5 μm. The diameter of terahertz focal area is 1 mm，which is much smaller than the thin-film size. All the measured transmittance and reflection spectra of as-fabricated MXene thin films are automatically calculated with removing the background signals. The morphology of MXene nano-flakes is captured by Zeiss® Gemini™ 500 SEM. To prevent unwanted discharge，a thin carbon layer is sputtered on the surface of the MXene thin-film by a Safematic® CCU-010 HV high compact vacuum coating system. Images were obtained by a secondary electron detector，with a working distance of about 5 mm and a 5 kV acceleration voltage. The detailed images on the lateral edge of as-fabricated MXene nano-flakes are recorded by Thermo Scientific Talos™ transmission electron microscope（TEM）. The AFM microscopy was performed on Smart SPM by AIST-NT，Diluted MXene nano-flakes are dispersed onto the mica plate for AFM measurement X-ray photoelectron spectroscopy（XPS）was performed on a X-ray photoelectron spectrometer（Thermo fisher Scientific，K-Alpha，USA）apparatus using a Al Kα X-ray source to investigate their surface electronic states.

The morphology of as-fabricated MXene flakes is conducted by AFM and SEM and TEM respectively. The Fig. 2（a）shows the Ti₃C₂T_x，Ti₃C₂T_x-C₈H₁₈，Ti₃C₂T_x-C₁₂H₂₆ MXene flakes are dissolved in the aquesous solution. As such，one can find out the single flakes obviously. The surface roughness of single flakes are measured by the AFM shonw in Fig. 2（b）. The alkane grafting increase the surface roughness of single flakes of Ti₃C₂T_x MXene. Figure 2（c）shows the cross-section of drop-casted MXene thin-films. The average thickness of all the thin-film is about（15±5）μm. The detailed surface morphology of Ti₃C₂T_x，Ti₃C₂T_x-C₈H₁₈，Ti₃C₂T_x-C₁₂H₂₆ MXene flakes are shown in Fig. 2（d）.

An obvious boundary exists in between the adjacent flakes of Ti₃C₂T_x MXene. A zoom-in insight on the flake shape shows straight lineshaped boundary of the flakes. However，the Ti₃C₂T_x-C₈H₁₈ and Ti₃C₂T_x-C₁₂H₂₆ showing the assembly of flakes mono-sheets seems to be crumpled shapes with numerous ridges and rough surface. Such a wrinkled morphology of Ti₃C₂T_x flakes due to alkane grafting. A zoom-in insight on the flake shape shows curved grain-boundary of the flakes. Meanwhile，the whole flakes mono-sheet seems to be in tensile or compressive deformation. Especially，the lateral edge of Ti₃C₂T_x-C₁₂H₂₆ exhibits folds along the lateral edge compared to the local lateral edge spikes of Ti₃C₂T_x-C₈H₁₈ MXene flakes. A deep insight on the curved deformation are achieved by TEM at nanometer scale，of which the electrons beam is focused on the lateral edge of aforementioned MXene flakes. As shown in Fig. 2，the edge of Ti₃C₂T_x MXene is a flat，while the lateral edge of Ti₃C₂T_x-C₈H₁₈ flakes is obviously folded. To the Ti₃C₂T_x-C₁₂H₂₆ flakes，a stronger edge-folding almost wrap up the flakes from the corner. Such a curvature introduced into 2D mono-sheet flakes changes the spatial distribution of strain so as to increase the roughness of MXene thin-film. This phenomenon is similar to the textured graphene of curved-flakes. Compared to a planar surface，a rough surface has a higher specific surface area according to the Brunauer–Emmett–Teller（BET）theory^［13］.

Since the MXene speciemen are coated with carbon for SEM measurement，the carbon ratio has to be excluded. Hytrogen is too light to be recorded in our EDS measurement. The aluminium is the residue of MAX phase precursor. Interestingly，the surface functional element T_x（-F，-O）oxygen-fluorine ratio（O：F）is changed from 2：1（Ti₃C₂T_x）up to 4：1（Ti₃C₂T_x-C₈H₁₈）and 7：1（Ti₃C₂T_x-C₁₂H₂₆）. As such，the alkanes occupy the atomic site of flurione so as to be grafted onto the surface of Ti₃C₂T_x MXene nano-flakes. Oxygen termination play a significant role in bridging nearby titanium between the Ti₃C₂T_x MXene nano-flakes. However，both C₈H₁₈ and C₁₂H₂₆ are straight-chain alkanes，of which the carbon-chain length is much longer than the oxygen bond length. As such，the alkane on the surface of one MXene nano-flake forms interlayer repulsion opposite to the force of oxygen bond resulting in a compressive/tentile deformation of alkane-grafted Ti₃C₂T_x MXene as well as the edge-folding effect.

The IR transmission and reflection spectra of the as-fabricated Ti₃C₂T_x MXene thin-film are presented in Fig. 3. Clearly，The transittance of Ti₃C₂T_x MXene，Ti₃C₂T_x-C₈H₁₈ and Ti₃C₂T_x-C₁₂H₂₆ MXene are almost close to the lowest detection limit of FT-IR since there is only background noise. However，the reflection spectra of as-fabricated Ti₃C₂T_x MXene thin-film shows a distinct larger reflectance than the other two types of alkane-grafted MXenes，which is presented in Fig. 3. The reflectance of Ti₃C₂T_x MXene is from 45% to 70% in the wavelength range from 2 μm to 4.8 μm. However，that of Ti₃C₂T_x-C₈H₁₈ MXene is below 3%，while that of Ti₃C₂T_x-C₁₂H₂₆ MXene is even below 1%. All the as-fabricated Ti₃C₂T_x MXene thin-film shows a climbing behavior in the reflection spectrum as a function of wavelength. It can be found that the reflection ratio decreases at a factor of 10 in the entire spectrum. According to the optics principle，the strong attenuation of transmittance and reflectance implies a dramatically increase of optical absorption in measurement，which obeys the regulation as below^［13］：

where R，A，and T are the fractions of the power which are reflected，absorbed，and transmitted correspondingly. Therefore，the optical absorption of samples can be extracted from above equation，as illustrated in Fig. 3.

Above the IR absorption of intrinsic Ti₃C₂T_x MXene from 55% to 29% with the wavelength increasing from from 2 μm to 4.8 μm. However，the IR absorption of Ti₃C₂T_x-C₈H₁₈ and Ti₃C₂T_x-C₁₂H₂₆ MXene are both beyond 99%. The slope of the IR absorption function of intrinsic Ti₃C₂T_x MXene decreases with the wavelength climbing up. To the Ti₃C₂T_x-C₈H₁₈ and Ti₃C₂T_x-C₁₂H₂₆ MXene，however，their IR spectra becomes very much flat compared to that of the intrinsic Ti₃C₂T_x MXene. At this point，one can summarize that the alkane graft enhances the IR absorbance by reducing the reflectance of MXene thin-film. The origin of absorption enhancement can be revealed from the morphologic graphics. The flake tensile or compressione as well as the edge-floding of the flakes enlarge the specific surface area for IR absorption，which enhances the effective area for the interaction of IR photon and MXene. These curvature increase the surface roughness inducing a diffusion reflection，which is responsible for the energy loss of specular reflection. Furthermore，the skeleton molecular vibration of carbon-chain maybe contribute to the enhancement of absorption at mid-wavelength infrared region. The spectra of simple alkanes are characterized by absorptions due to C-H stretching and bending The IR spectrum of octane is shown strong bands in the 3 000-2 850 cm^-1 region due to C-H stretch. The C-H scissoring（1 470 cm^-1），methyl rock（1 383 cm^-1），and long-chain methyl rock（728 cm^-1）are noted on this spectrum^［14］. All these vibration are locate in the mid-wavelength infrared region from 3 μm to 5 μm. Since most organic compounds have these features，these C-H vibrations are usually not noted when interpreting a routine IR spectrum. However，the C=C stretching and bending bands are either too weak or of too low a frequency to be detected in IR spectroscopy.

The versatility of MXenes at infrared wavelengths provides a platform for developing MXene-based smart，flexible devices and wearables capable of selective and localized thermal management，aiming at radiative heating/cooling，IR identification，photothermal conversion，and thermal imaging. Alkane graft increases the absorbance of MXene by reducing the reflectance. The edge-folding of the alkane decorated MXene increases the specific surface area，which contribute to the enhancement of absorption at mid-wavelength infrared region. The skeleton molecular vibration of carbon-chain maybe contribute to the enhancement of absorption at mid-wavelength infrared region.