Solid state sensors utilizing diamond nitrogen-vacancy (NV) centers are a promising sensing platform that can provide high sensitivity and spatial resolution at high precision. Such sensors have been realized in bulky laboratory-based forms; however, practical applications demand a miniaturized, portable sensor that can function in a wide range of environmental conditions. Here, we demonstrate such a diamond NV magnetic field sensor. The sensor head fits inside a 11×7×7 cm3 3D-printed box and exhibits sub-10 nT/Hz sensitivity over a 125 Hz bandwidth. We achieve efficient fluorescence collection using an optical filter and diode in contact with the diamond, which is cut at the Brewster angle to maximize the coupling of 532 nm pump light. We discuss the potential of this flexible approach to achieve sub-nT/Hz shot noise limited sensitivity suitable for detection of a wide range of low-level magnetic fields, particularly those from electrical power systems and from biological sources.

Semiconducting nanowires, unlike bulk, can be grown in both wurtzite and zincblende crystal phases. this unique feature allows for growth and investigation of technologically important and previously unexplored materials, such as wurtzite AlGaAs. Here we grow a series of wurtzite AlGaAs nanowires with Al content varying from 0.1 to 0.6, on silicon substrates and through a comparative structural and optical analysis we experimentally derive, for the first time, the formula for the bandgap of wurtzite AlGaAs. Moreover, bright emission and short lifetime of our nanowires suggest that wurtzite AlGaAs is a direct bandgap material.Polytypism 1 is an exceptional property of nanowires and a new degree of freedom which enables the engineering of the electronic structure without change of material. For example, today's atomically-precise control over the crystal-phase switching in nanowires 2,3 allows to grow strain-free polytypic formations along the growth axis 4,5 , even small enough to form quantum dots 6,7 . The wurtzite phase is not observable at ambient conditions in bulk of any A III B V materials except for nitrides, while it can be obtained in nanowires. For this property and its technological implications, a great deal of attention has been drawn, in recent years, to nanowires system from scientific community 8-10 . However, for designing of novel structures and devices, knowledge of bandgaps and band alignments of the different crystal phases of new materials is crucial.In particular, Al X Ga 1-X As nanowires provide a promising platform for fabrication of advanced devices. For example, adding the Al component to the widely studied GaAs 11,12 allows to tune the emission in a wide range of wavelengths while, AlGaAs, having higher energy than GaAs, allows the combination of these two materials to fabricate strain-free quantum devices 13 .However, the knowledge about wurtzite AlGaAs is limited in the literature [14][15][16][17] , and is mainly grown as a shell around wurtzite GaAs core 15,16 . Importantly, the bandgap of wurtzite AlGaAs was neither predicted theoretically nor measured experimentally.In this work, we grow wurtzite AlGaAs nanowires, in a wide range of Al content x, and we present a comparative optical and structural study, empirically revealing the trend for the bandgap of wurtzite Al X Ga 1-X As. We grow our samples by Au-catalyzed vapor-liquid-solid technique in a molecular beam epitaxy (MBE) reactor (see methods section for details) obtaining high crystalline quality structures with any chosen Al content.

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