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incompatibilities with silicon and modern semiconductor processes. [5] Since the discovery of ferroelectricity in HfO 2 -based thin films in 2011, [6] fluorite-structure binary oxides have attracted considerable interest as they are compatible with complementary metal-oxide-semiconductor (CMOS) processes. [7] Accordingly, HfO 2based ferroelectric memory has received significant attention in recent years, [1,8,9] primarily focused on charge-based ferroelectric random access memory (FeRAM) and ferroelectric field effect transistors (FeFETs). [2,10] Meanwhile, resistiveswitching materials-which exhibit electrically-induced resistance changes in metal-dielectric-metal junctions or heterostructures with multi-dielectric barriershave emerged as promising candidates for novel beyond-CMOS data-centric computing paradigms. [11][12][13] In this context, ferroelectric tunnel junctions (FTJs) present a promising energy-efficient resistive switching memory [12,13] as FTJs exploit the ferroic polarization functionality of the insulating barrier. [14] Voltage-controlled polarization-dependent tunneling through the ferroelectric layer (tunnel electroresistance, TER) can yield much larger ON/OFF conductance ratios [15,16] than, for example, current-controlled magnetic tunnel junctions, [12] another two-terminal tunneling resistive switching device.A critical requirement for FTJs is to achieve a sufficiently high tunneling current (J ON ) at the ON state to ensure that a scaled device can be read rapidly, while still exhibiting a large TER ((J ON -J OFF )/J OFF × 100%). [13] Considering the large band gap of HfO 2 (≈6 eV), the thickness of HfO 2 in the FTJ will need to be reduced to the ultrathin limit for adequate tunnel current. Tunnel junctions implementing CMOS-compatible HfO 2 -based ferroelectric barriers have been recently demonstrated, [17][18][19] but even three nanometer Zr-doped HfO 2 (Zr:HfO 2 ) barriers were found to be too thick to obtain nano-ampere level current in micron-sized capacitors. [20] Therefore, high ON current is a critical consideration; however, the increased ON state current from an ultrathin barrier will coincide with an increased OFF state current. For array-level implementations, where sneak leakage paths can lead to increased power consumption, selector devices may be required in conjunction with the FTJ memory elements to reduce such sneak currents. [13] Here, we demonstrate FTJs utilizing one nanometer Zr:HfO 2 as the ferroelectric barrier, grown by atomic layer deposition (ALD) directly on silicon, thereby scaling down the tunnel barrier ABSTRACT: In ferroelectric materials, spontaneous symmetry breaking leads to a switch-able electric polarization, which offers significant promise for nonvolatile memories. In particular, ferroelectric tunnel junctions (FTJs) have emerged as a new resistive switching memory which exploits polarizationdependent tunnel current across a thin ferroelectric barrier. This work integrates FTJs with com-plementary metal-oxide-semiconductor-compatible Zr-doped HfO 2 (Zr...

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A great deal of attention has been paid to brown carbon aerosol in the troposphere because it can both scatter and absorb solar radiation, thus affecting the Earth's climate. However, knowledge of the optical and chemical properties of brown carbon aerosol is still limited. In this study, we have investigated different aspects of the optical properties of brown carbon aerosol that have not been previously explored. These properties include extinction spectroscopy in the mid-infrared region and light scattering at two different visible wavelengths, 532 and 402 nm. A proxy for atmospheric brown carbon aerosol was formed from the aqueous reaction of ammonium sulfate with methylglyoxal. The different optical properties were measured as a function of reaction time for a period of up to 19 days. UV/vis absorption experiments of bulk solutions showed that the optical absorption of aqueous brown carbon solution significantly increases as a function of reaction time in the spectral range from 200 to 700 nm. The analysis of the light scattering data, however, showed no significant differences between ammonium sulfate and brown carbon aerosol particles in the measured scattering phase functions, linear polarization profiles, or the derived real parts of the refractive indices at either 532 or 402 nm, even for the longest reaction times with greatest visible extinction. The light scattering experiments are relatively insensitive to the imaginary part of the refractive index, and it was only possible to place an upper limit of k ≤ 0.01 on the imaginary index values. These results suggest that after the reaction with methylglyoxal the single scattering albedo of ammonium sulfate aerosol is significantly reduced but that the light scattering properties including the scattering asymmetry parameter, which is a measure of the relative amount of forward-to-backward scattering, remain essentially unchanged from that of unprocessed ammonium sulfate. The optical extinction properties in the mid-IR range (800 to 7000 cm(-1)) also showed no significant changes in either the real or the imaginary parts of the refractive indices for brown carbon aerosol particles when compared to ammonium sulfate. Therefore, changes in the optical properties of ammonium sulfate in the mid-IR spectral range due to reaction with methylglyoxal appear to be insignificant. In addition to these measurements, we have characterized additional physicochemical properties of the brown carbon aerosol particles including hygroscopic growth using a tandem-differential mobility analyzer. Compared to ammonium sulfate, brown carbon aerosol particles are found to have lower deliquescence relative humidity (DRH), efflorescence relative humidity (ERH), and hygroscopic growth at the same relative humidities. Overall, our study provides new details of the optical and physicochemical properties of a class of secondary organic aerosol which may have important implications for atmospheric chemistry and climate.